www/qt6-webengine: Fix build on recent -CURRENT

Due to recent changes in string.h, the regex in our local patch for the
bundled nasm to check whether mempcpy(3) was available no longer worked.
This caused it to use its own implementation which caused an error due
to a declaration conflict.

Since all currently supported versions of FreeBSD have mempcpy(3), this
patch is no longer needed.

PR: 280376