piglit/util: remove optimization for extension string query

When Mesa is compiled with option 'draw-use-llvm=true', this is
causing trouble for egl_khr_fence_sync test that runs multiple subtests
that check for extension support. Valgrind shows invalid read in this
case:

 ==52394== Invalid read of size 1
 ==52394==    at 0x484C784: strcmp (vg_replace_strmem.c:940)
 ==52394==    by 0x4B54B89: piglit_is_extension_in_array (piglit-util.c:162)
 ==52394==    by 0x4A6D8A9: piglit_is_extension_supported (piglit-util-gl.c:130)

Destroying the list and allocating it again makes it work.

Closes: https://gitlab.freedesktop.org/mesa/piglit/-/issues/100
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>