xfree86: remove leftover ev56.c source files

Commit 166ac29 replaced the function calls in these files with inline
functions in compiler.h and updated the configure scripts & meson builds
to stop compiling them, but missed actually removing the now unused sources.

Fixes: 166ac29 ("xfree86: Inline xf86{Read,Write}Mmio{8,16,32} on alpha")
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2122>

Author: alanc