libsnapshot: delete WaitForMerge.

Now that update_engine is responsible for initiating the
merge, WaitForMerge function becomes useless.

Bug: 147696014
Test: compiles
Change-Id: I3779bdf9dd8d1716238e21d09acf0499e3fc90a3
Merged-In: I3779bdf9dd8d1716238e21d09acf0499e3fc90a3
5 files changed