Clean up some <dfn>s that were not referenced (#465)

This fixes Bikeshed warnings like

    LINT: Unexported dfn that's not referenced locally - did you mean to export it?
    <dfn bs-line-number="550" data-lt="reading value" data-dfn-type="dfn" id="reading-value" data-noexport="by-default">values<a href="#reading-value" class="self-link"></a></dfn>
    <dfn bs-line-number="550" data-lt="reading value" data-dfn-type="dfn" id="reading-value" data-noexport="by-default">values<a href="#reading-value" class="self-link"></a></dfn>

Some definitions used to be referenced years ago in previous versions of the
spec text.

While here, also fix

    The var 'latest reading' (in algorithm 'update latest reading') is only used once.
    If this is not a typo, please add an ignore='' attribute to the <var>.

by referencing the dfn rather than using `latest reading` as a variable.

Related to #444.

index.bs

@@ -547,7 +547,7 @@ A [=device sensor=] measures a physical quantities
 and provides a corresponding <dfn export>sensor reading</dfn>
 which is a source of information about the environment.
 
-Each [=sensor reading=] is composed of the <dfn lt="reading value">values</dfn>
+Each [=sensor reading=] is composed of the values
 of the physical quantity measured by the [=device sensor=]
 at time <var ignore>t<sub>n</sub></var> which is called the <dfn>reading timestamp</dfn>.
 
@@ -558,8 +558,8 @@ a reference frame for the [=device sensor=]'s [=sensor readings=].
 A [=device sensor=] that provides such [=sensor readings=]
 is referred to as <dfn export>spatial sensor</dfn>.
 
-A [=spatial sensor=] can be <dfn>uniaxial</dfn>, <dfn>biaxial</dfn>,
-or <dfn>triaxial</dfn>, depending on the number of orthogonal axes in
+A [=spatial sensor=] can be uniaxial, biaxial,
+or triaxial, depending on the number of orthogonal axes in
 which it can perform simultaneous measurements.
 
 Scalar physical quantities (e.g. temperature) do not require
@@ -645,11 +645,11 @@ for the given [=sensor types=] they are targeting.
 through a process called <dfn>sensor fusion</dfn>.
 This process provides [=high-level|higher-level=] or
 more accurate data (often at the cost of increased latency).
-For example, the [=sensor readings|readings=] of a [=triaxial=] magnetometer
+For example, the [=sensor readings|readings=] of a triaxial magnetometer
 needs to be combined with the [=sensor readings|readings=] of an accelerometer
 to provide a correct bearing.
 
-<dfn>Smart sensors</dfn> and <dfn>sensor hubs</dfn>
+Smart sensors and sensor hubs
 have built-in compute resources which allow them
 to carry out [=calibration=] and [=sensor fusion=] at the hardware level,
 freeing up CPU resources and lowering battery consumption in the process.
@@ -1402,7 +1402,7 @@ to {{SensorErrorEventInit}}.
     1.  Let |type| be |sensor|'s associated [=sensor type=].
     1.  If |type|'s [=threshold check algorithm=] is defined, then:
         1.  Let |result| be the result of invoking |type|'s [=threshold check algorithm=]
-            with |reading| and |latest reading|.
+            with |reading| and |sensor|'s [=latest reading=].
         1.  If |result| is false, then abort these steps.
     1.  [=map/For each=] |key| → <var ignore>value</var> of [=latest reading=].
         1.  [=map/Set=] [=latest reading=][|key|] to the corresponding