Deprecate QT_NO_CAST_TO_ASCII

It seems like the last actual uses of QT_NO_CAST_TO_ASCII were removed in qtbase/d7ccd8cb4565c8643b158891c9de3187c1586dc9. There's a use still in qstringbuilder.h. Apart from that, the real QString::operator const char*() is in Qt 4 as QT3_COMPAT, but no longer in Qt 5.0.

Acceptance criteria:

• Remove last behaviour change under QT_NO_CAST_TO_ASCII from qstringbuilder.h, as if it is unconditionally defined
• remove all in-tree users
• Deprecate the macro
  • in the docs
  • in code (there's a way to deprecate macros, saw it on the old C++11 atomic init macro in libc++, at least for Clang, then)