Removed kapua-external module references from documentation

[Coduz]

This PR remove all references of kapua-external module from the documentation.
This was an old module that has been removed.

Related Issue
This PR closes #3196

Description of the solution adopted
Just removed references.

Screenshots
None

Any side note on the changes made
None

[codecov]

Codecov Report

Merging #3214 (b8d9a93) into develop (0620420) will increase coverage by 1.28%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #3214      +/-   ##
=============================================
+ Coverage      55.30%   56.58%   +1.28%     
- Complexity      2502     2679     +177     
=============================================
  Files           1273     1273              
  Lines          26996    26996              
  Branches        2340     2340              
=============================================
+ Hits           14929    15277     +348     
+ Misses         11177    10799     -378     
- Partials         890      920      +30     

Impacted Files	Coverage Δ	Complexity Δ
...client/transport/TransportElasticsearchClient.java	1.61% <0.00%> (-80.57%)	0.00% <0.00%> (ø%)
...ce/management/commons/call/DeviceCallExecutor.java	62.79% <0.00%> (-4.66%)	0.00% <0.00%> (ø%)
.../DeviceManagementOperationRegistryServiceImpl.java	66.07% <0.00%> (-3.58%)	12.00% <0.00%> (-2.00%)
...atch/persistence/jpa/JpaExecutionInstanceData.java	87.80% <0.00%> (-2.44%)	22.00% <0.00%> (-1.00%)
...ua/broker/core/listener/DeviceMessageListener.java	60.46% <0.00%> (-2.33%)	0.00% <0.00%> (ø%)
...ce/datastore/internal/mediator/DatastoreUtils.java	71.48% <0.00%> (+0.38%)	69.00% <0.00%> (ø%)
...pse/kapua/commons/service/internal/ServiceDAO.java	74.84% <0.00%> (+1.25%)	0.00% <0.00%> (ø%)
...apua/job/engine/jbatch/JobEngineServiceJbatch.java	51.85% <0.00%> (+1.85%)	8.00% <0.00%> (ø%)
...nnection/internal/DeviceConnectionServiceImpl.java	69.31% <0.00%> (+2.27%)	17.00% <0.00%> (+1.00%)
...e/kapua/service/job/step/internal/JobStepImpl.java	70.37% <0.00%> (+3.70%)	11.00% <0.00%> (+1.00%)
... and 32 more