configure: remove non-functional --with-opencl-libdir

configure: remove non-functional --with-opencl-libdir

The parameter used to control where the gallium pipe-drivers
were installed, but was broken since

commit 45270fb0fd1abd7619933c2845f9dc74cdfbe6fd
Author: Matt Turner <mattst88@gmail.com>
Date:   Thu Sep 13 10:45:01 2012 -0700

    targets/pipe-loader: Convert to automake

Considering that nowadays the pipe-drivers can be used by
more than just the opencl target, even fixing this up will
not be the best idea.

Cc: Matt Turner <mattst88@gmail.com>
Cc: Francisco Jerez <currojerez@riseup.net>
Buzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61415
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>