meson: eliminate libsystemd_journal_internal and use libsystemd_internal less

meson: eliminate libsystemd_journal_internal and use libsystemd_internal less

This simplifies things and leads to a smaller installation footprint.
libsystemd_internal and libsystemd_journal_internal are linked into
libystemd-shared and available to all programs linked to libsystemd-shared.
libsystemd_journal_internal is not needed anymore, and libsystemd-shared
is used everwhere. The few exceptions are: libsystemd.so, test-engine,
test-bus-error, and various loadable modules.