tcg: simplify !CONFIG_TCG handling of tb_invalidate_*

There is no need for a stub, since tb_invalidate_phys_addr can be excised
altogether when TCG is disabled.  This is a bit cleaner since it avoids
using code that is clearly specific to user-mode emulation (it calls
mmap_lock/unlock) for the !CONFIG_TCG case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>