[REG2.063] Disable enhancement feature 7444

[The enhancement 7444](http://d.puremagic.com/issues/show_bug.cgi?id=7444) has been introduced in 2.063. But now, there's some issues which are not easy to immediately resolve.

I'd suggest to disable the feature 7444 in 2.064 release.

Existing issues:

[Issue 11276](http://d.puremagic.com/issues/show_bug.cgi?id=11276) - Spurious "explicit slice assignment ...[] is better" warning message in Phobos
To enforce slice + assign syntax `a[] = b[]` may cause problems with generic code.

[Issue 11244](http://d.puremagic.com/issues/show_bug.cgi?id=11244) - Invalid warning about slice assignment
A case yet difficult to make it clear - combination with operator overloading.