Clean up WINVER, _WIN32_WINNT macros for MinGW.

Define WINVER, _WIN32_WINNT as 0x501 (Windows XP) in qt_windows.h.
Remove definitions of the same/lower versions and unneeded
definitions in other places. Remove definition for Borland compiler.

Task-number: QTBUG-51673
Change-Id: I2a344a7f7cf78b2afbf45dcdf8bf2a19b93f0a07
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>