Additionally I erased the remains of the old mysql C++ connector.

Also update the code where the clang plugins were unhappy after moving
the mysqlc module.

Remove mysql-cpp-connector external.

Change-Id: I771cc7afacf5012fc349afb6b36828fb0fee5a66
Reviewed-on: https://gerrit.libreoffice.org/59200
Tested-by: Jenkins
Reviewed-by: Tamás Bunth <btomi96@gmail.com>