mh-mingw: drop unused BOOT_CXXFLAGS variable

gcc's build system has BOOT_CFLAGS and various STAGE<N>_C{,XX}FLAGS
variables. BOOT_CXXFLAGS is not handled anywhere.

config/

	* mh-mingw: Drop assignment of unused BOOT_CXXFLAGS variable.

config/mh-mingw

 # Add -D__USE_MINGW_ACCESS to enable the built compiler to work on Windows
 # Vista (see PR33281 for details).
 BOOT_CFLAGS += -D__USE_MINGW_ACCESS -Wno-pedantic-ms-format
-BOOT_CXXFLAGS += -D__USE_MINGW_ACCESS -Wno-pedantic-ms-format
 CFLAGS += -D__USE_MINGW_ACCESS
 CXXFLAGS += -D__USE_MINGW_ACCESS
 STAGE1_CXXFLAGS += -D__USE_MINGW_ACCESS