releasetools: Remove the dead code for blockimgdiff_version <= 2.

The code is on infeasible path since we already have assertions in
common.BlockDifference().

Also remove the dead code that checks for OPTIONS.info_dict, as we
already set that in ota_from_target_files.main(), for both of A/B and
non-A/B.

Test: Generate incremental OTAs w/ and w/o the CL, and get identical
      packages.
Change-Id: Ifb8fc101e78f5ce58c60c8e49028b66ce0d20246
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 829b8db..12e757d 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -1431,11 +1431,9 @@
     self.disable_imgdiff = disable_imgdiff
 
     if version is None:
-      version = 1
-      if OPTIONS.info_dict:
-        version = max(
-            int(i) for i in
-            OPTIONS.info_dict.get("blockimgdiff_versions", "1").split(","))
+      version = max(
+          int(i) for i in
+          OPTIONS.info_dict.get("blockimgdiff_versions", "1").split(","))
     assert version >= 3
     self.version = version