Dev: Remove `master` branch

[kenjis]

The master branch only complicates the work and has no benefit.

Currently, the master branch is diverged, v1.2.0 is not included in master or develop:

[datamweb]

Deleted branch master was the best choice in Shield. Simplify and summarize processes.

[kenjis]

I don't know why master was diverged, but I would like to do the following:

$ git checkout master 
$ git merge v1.2.0 
Merge made by the 'ort' strategy.
$ git checkout develop 
$ git merge master 
Updating 78caaaf..9d4801f
Fast-forward
$ git branch -D master

and will be like this:

[kenjis]

Done.

Removed master 78caaaf