c++: remove LAMBDA_EXPR_CAPTURES_THIS_P

This unused accessor is just a simple alias of LAMBDA_EXPR_THIS_CAPTURE
and contrary to its documentation doesn't use TREE_LANG_FLAG_0.  Might
as well remove it.

gcc/cp/ChangeLog:

	* cp-tree.h (LAMBDA_EXPR_CAPTURES_THIS_P): Remove.

Reviewed-by: Jason Merrill <jason@redhat.com>