[CI] Run the functional tests when debug is enabled

Summary:
Now that the functional tests are passing with debug enabled, the
ENABLE_DEBUG flag from the build script is no longer required.

Test Plan:

export CONFIGURE_FLAGS="--enable-debug --disable-ccache"
./contrib/teamcity/build.sh

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3740