Remove mirrored libevent1 and libpcre2 OSSEC dependencies

[eloquence]

In freedomofpress/securedrop-apt-test#40 and the corresponding private repo prod PR we added the OSSEC dependencies libevent1 and libpcre2 to apt-test.freedom.press and apt.freedom.press, respectively, in order to resolve #5213. This should only be required for upgrades, not for a fresh install which should automatically pull in all the required dependencies.

We've publicly communicated that we will no longer provide an upgrade path from 1.2.2 with the release of SecureDrop 1.5.0, so we can remove these mirrored packages as part of the 1.5.0 release.