Revert "Register connection types explicitly if not already registered"

This reverts commit 3550d22da67ab828ad83f18621882aaeb853f256.

The restoration of type registration in qtbase with commit
3bb40a9706d51ef0582e44827dc47bc6232211b6 makes the explicit fallback
unnecessary.

The tst_proxy test emits warnings, both with and without this revert.
The tests pass, so the impact of this is unclear and requires further
investigation that is out of scope for this change.

QWARN  : ProxyTest::testProxy(dynamicApi, with proxy, dynamicRep)
         Failed to register enum "Engine::EngineType" (this name is
         already in use).
PASS   : ProxyTest::testProxy(dynamicApi, with proxy, dynamicRep)
QWARN  : ProxyTest::testProxy(sourceApi, with proxy, dynamicRep)
         Failed to register enum "Engine::EngineType" (this name is
         already in use).
PASS   : ProxyTest::testProxy(sourceApi, with proxy, dynamicRep)

Pick-to: 6.12
Change-Id: I0d74914a411d9066c19daa72c51ca3bd5317fe9d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>