Resolve "doxygen warning/error: documented empty return type of FFT plan functions"

Description

Removes instances of documented None/void returns that newer doxygen versions (tested with 1.9.3 currently on conda-forge) don't seem to like. For comparison, the CI seems to currently run with 1.8.13.

• 4 instances of @return None in lal/lib/fft which should be ok to just remove, matching other destructor functions
• 1 claim of an actual return in lalinference while that function is actually also void. This seems to just seek through a file and indeed not return anything.
• 1 claim of an actual return in lalpulsar/bin/HeterodyneSearch/ppe_roq.c while that function is actually also void - @matthew-pitkin for this one it wasn't actually clear to me where the stuff the function calculates ends up, as there is no [in/out] argument either; is it just written to a file? Help string could be amended maybe.
• 3 more in lalpulsar/bin/TwoSpect/ which seem to be leftovers from changing the return type a long time ago and the remaining documentation is clear enough (cc @evan-goetz )

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

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Closes #570 (closed)