mysql: revert PKG_SOURCE_URL previous patch

This patch reverts previous patch that fixes already
fixed non-functional repos. MySQL archives repo is
duplicit and planetmirror is offline.

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@35802 3c298f89-4303-0410-b956-a3cf2f4a3e73