Added by 17ab095a ("add libdir/exec_prefix to xcb-xproto.pc") with the
rationale of using it to find the python path, but later commits bd7a481a
("pkg-config: Add sysroot prefix") and be9c255b

 ("Adjust to automake
1.16.4 changes") make this directly accessible.

By removing libdir from the pkgconfig file, the file becomes identical
for different ABIs.

Signed-off-by: Matt Turner <mattst88@gmail.com>