[PATCH]Remove obsolete macro CHECKFLOATVAL in btree_gist

Hi

Commit 6bf0bc842 replaced float.c's CHECKFLOATVAL() macro with static inline subroutines,
but the fix introduced a performance regression as Tom Lane pointed out and fixed at 607f8ce74.

Found obsolete CHECKFLOATVAL usage in contrib/btree_gist, and tried to fix it according to 607f8ce74.
The attached patch has been testified in master. All tap tests passed.

Regards,
Tang