Remove CHECK_IF_CURL_DEPENDS_ON_RTMP to build keyring_vault for unconditional test

On Ubuntu Bionic (where CURL library depends on RTMP library) keyring_vault unit tests can be build and run without any issues (all clean including ASan and Valgrind). However, by default they are disabled by CHECK_IF_CURL_DEPENDS_ON_RTMP CMake macro.

To test, just disable CHECK_IF_CURL_DEPENDS_ON_RTMP() calls in

plugin/keyring_vault/keyring_vault-test/CMakeLists.txt
unittest/gunit/keyring_vault/CMakeLists.txt

build and run

./mysql-test/mtr --debug-server main.1st --unit-tests

Consider eliminating this macro and build keyring_vault unit tests unconditionally.