Don't delete the kernel when pre-compiled.

Because we'll be re-using that same kernel instance for subsequent runs.

(cherry picked from commit 80842a50b1580b9a6da3a2c39a88a72b5b98d278)

Conflicts:
	sc/source/core/opencl/formulagroupcl.cxx

Change-Id: I08b6df65550cecfa48f937b163fb1c5b20ed8ac9
Reviewed-on: https://gerrit.libreoffice.org/8538
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>