ewaol-tests/runtime-integration-tests.bbclass: Drop upstreamed ptest RDEPENDS

The dependency of ptest-runner on libgcc has been upstreamed, so drop it from
the runtime-integration-tests bbclass.

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I7be2452650450b0ff4c86f39a1497fe3271c14ae