bts/osmo-bts.cfg: reduce OML logging verbosity, degrade to notice

Average size of osmo-bts.log after running ttcn3-bts-test is ~178 MB,
and more than 52% (!) of it is the OML logging messages.  Other than
making the logfile bigger, bursts of OML messages make it really hard
to follow the output of osmo-bts when running tests natively.

Change-Id: Ib5346a3ca42917169e195f905472ccea7579e5eb
diff --git a/bts/osmo-bts.cfg b/bts/osmo-bts.cfg
index 5609212..f581be0 100644
--- a/bts/osmo-bts.cfg
+++ b/bts/osmo-bts.cfg
@@ -18,7 +18,6 @@
 !
  logging level set-all notice
  logging level rsl info
- logging level oml info
  logging level meas info
  logging level pag info
  logging level l1c info