virtual: l1_if.c: Remove unneeded warning message
virtual: l1_if.c: Remove unneeded warning message

Commit 5eb17e28acdd6fba22a1f2e60f4d55aaef18b47a added this warning when
implementing the full function. However, other backends seem to be also
passing the primitive from the stack to l1sap_up without any visible issues.
Furthermore, according to the documentation of l1sap_up, it takes ownership of the
msgb, but doesn't take ownserhip of the prim itself.

Change-Id: I45fe40e3377eac999d1dac5356678195381d94ca

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts