Remove branch change notice from README

Removes the notice regarding branch name changes from the main
README file. Also changes the default branch to 'master' in the
.gitreview file.

Change-Id: I77f7115751a2b469f13dd51c159ec7d3f2fc1a12

.gitreview

@@ -2,4 +2,4 @@
 host=review.opendev.org
 port=29418
 project=airship/treasuremap.git
-defaultbranch=v2
+defaultbranch=master

README.md

@@ -17,11 +17,3 @@ We also encourage new contributors and operators alike to join us in our
 
 You can learn more about Airship on the [Airship
 wiki](https://wiki.openstack.org/wiki/Airship).
-
-## Please Note:
-The branching structure of this repository will be changing to the following:
-* The 'v2' branch will become the 'master' branch
-* The current 'master' branch will become a new 'v1.9' branch
-* The current 'v1.8' branch will remain the same
-
-These changes are planned to take effect on Thursday, April 1, 2021, or slightly thereafter.