docs: remove stray reference to -fanalyzer-checker=taint [PR103533]

I missed this one in r14-5464-gcfaaa8b11b8429.

gcc/ChangeLog:
	PR analyzer/103533
	* doc/extend.texi: Remove stray reference to
	-fanalyzer-checker=taint.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>