Add a separate function to clear the IMSI filter to be used instead of
gbprox_set_patch_filter(cfg, NULL, ...). Albeit it fixes a Coverity
issue (Unchecked return value), it is a false positive, since the
return value is always 0 in these cases. Nevertheless it is more
obvious what happens when an explicit clear function is called. Using
NULL as filter argument of gbprox_set_patch_filter still clears the
filter.
Fixes: Coverity CID 1231255
Sponsored-by: On-Waves ehf
29805da6d1