Check that install file exists before unlink

This will remove spurious error messages in logcat such as:

08-02 00:27:21.580   600   600 E /system/bin/recovery-persist: Failed to
unlink /data/misc/recovery/last_install: No such file or directory

since the install file doesn't exist in the first place.

Bug: None
Test: m
Change-Id: Ifaa95729c50efae4e641286dfbe1718aceb5f50a