gbm: remove no-longer-needed dlopen-call

This is no longer needed, since 79a26b66 ("gallium/dri: always link
against shared glapi") and d884d8d0 ("egl/dri: link directly to
libglapi.so") landed. So let's remove it.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
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