pgm: remove explicit version of log4j and slf4j in pom.xml

pgm: remove explicit version of log4j and slf4j in pom.xml

The version is inherited from the master pom.xml so does not need
to be explicitly specified, unless the intent is to override the
version. This is not the case, though, since the versions are
specified with the properties defined in the master pom.xml.

Despite the fact that using the properties results in the versions
effectively not being overridden, Eclipse still warns about it,
seemingly due the fact that a value is given in the version field.

Change-Id: I163ede6689b9e2e7dd7b21f22b4068d638fa97aa
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>