Remove casting_syntax=both option from flowconfigs now that it is t…

…he default

Summary: Remove `casting_syntax=both` option from flowconfigs now that it is the default. Also remove some `exact_by_default=true` which has been `true` by default for a while.

Reviewed By: SamChou19815

Differential Revision: D54287088

fbshipit-source-id: fcca7e49b19bfe030a52d30a26eb433f6da9c156

.flowconfig

@@ -21,8 +21,6 @@ suppress_type=$FlowFixMeState
 suppress_type=$FlowFixMeEmpty
 suppress_type=$FlowExpectedError
 
-casting_syntax=both
-
 [lints]
 sketchy-null-number=warn
 sketchy-null-mixed=warn