patches: remove patch for __is_convertible

__is_convertible patch does not applies to 15.0.0 version.

It is possible to compile libcxx without this patch. Tested for
gcc-11.

This reverts the commit:
4dd2450: patches: Backport upstream fix for __is_convertible

Signed-off-by: Mamta Shukla <mamtashukla555@gmail.com>

Author: mamtashukla