distutils-r1.eclass: Do not apply patches if DISTUTILS_OPTIONAL is used

distutils-r1.eclass: Do not apply patches if DISTUTILS_OPTIONAL is used

Do not apply PATCHES and user patches (either via the EAPI 6 default or
pre-EAPI 5 code) when DISTUTILS_OPTIONAL is being used. In this case,
distutils functions are usually called conditionally, in a subdirectory,
while both PATCHES and user patches are usually intended to be applied
top-level.

There is no ebuild relying on distutils-r1_src_prepare applying patches
with DISTUTILS_OPTIONAL. In fact, there are ebuilds which work around
this behavior.