[msan] Remove non-working MSAN_LINUX_X86_64_OLD_MAPPING

r249754 (2015) added this macro (not set anywhere) for an escape hatch.
This old layout has been non-working for many years (Linux
arch/x86/include/asm/elf.h ELF_ET_DYN_BASE is outside the APP range):

    FATAL: Code 0x558547327980 is out of application range. Non-PIE build?
    FATAL: MemorySanitizer can not mmap the shadow memory.
    FATAL: Make sure to compile with -fPIE and to link with -pie.
    FATAL: Disabling ASLR is known to cause this error.
    FATAL: If running under GDB, try 'set disable-randomization off'.

Non-pie doesn't work either.