um: Remove broken highmem support

Highmem was always buggy and experimental on UML(i386).
In times where 64 bit computers are default we can
remove that experimental code.

Signed-off-by: Richard Weinberger <richard@nod.at>

diff --cc arch/um/Kconfig.um
Simple merge

diff --cc arch/um/include/asm/fixmap.h
Simple merge

diff --cc arch/um/include/asm/pgtable.h
Simple merge

diff --cc arch/um/kernel/mem.c
Simple merge

diff --cc arch/um/kernel/um_arch.c
Simple merge

diff --cc arch/x86/um/Makefile
Simple merge