Revert "px5g-standalone: provide px5g via PROVIDES"

This reverts commit cc66f819.

This commit causes opkg to install px5g-standalone instead of px5g when
installing luci-ssl. As luci-ssl depends on mbedtls, using
px5g-standalone makes no sense. Next to that, it creates deprecated SHA1
certificates. Revert the commit to avoid pxg5-standalone to be
installed by accident.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>