LALSimIMREOBNRv2HMROM.c variable set but not used error

Since !1608 (merged) the clang-dev nightly build has been failing with:

../../../../lalsimulation/lib/LALSimIMREOBNRv2HMROM.c:306:8: error: variable 'ret' set but not used [-Werror,-Wunused-but-set-variable]
  INT4 ret = XLAL_SUCCESS;
       ^
1 error generated.

The full build log is here

If the variable is not used it should be removed.

/cc @riccardo-sturani