mesalib: Remove patch to add demo programs

The patch from the BLFS book [1] breaks building out of the source
directory, so remove it, and build the demo programs in a separate
package. The directory `xdemos` would probably need to be added to
`Makefile.am`.

```
[BEE] make -j64 -C xdemos DEMOS_PREFIX=/usr
make: *** xdemos: No such file or directory.  Stop.
```

[1] http://www.linuxfromscratch.org/blfs/view/svn/x/mesa.html

mesalib.be0

@@ -15,7 +15,7 @@ SRCURL[0]="https://mesa.freedesktop.org/archive/mesa-${PKGVERSION}.tar.xz"
 ## Add URLs/pathes to patch files to the PATCHURL array.
 ## The sources will be patched in the order of the array.
 
-PATCHURL+=(http://www.linuxfromscratch.org/patches/blfs/svn/mesa-17.1.5-add_xdemos-1.patch)
+# PATCHURL+=()
 
 ###############################################################################
 ## Add filename patterns to the EXCLUDE array of files that should not