grl-inspect: Fix core keys extraction
This fixes incorrectly duplicated keys in grl-core-keys.h which affected the output of metadata in grl-inspect by removing the C comments prior to processing the file.
Before: $ sort grl-core-keys.h | uniq --repeated "GRL_METADATA_KEY_MB_RELEASE_GROUP_ID",
After: $ sort grl-core-keys.h | uniq --repeated $