removing master branch

[dom...@google.com]

hello!

15 months ago, we renamed the master branch to main.  shortly afterwards, i was asked to create a clone of the main branch called master to support some existing integrations.

it has been long enough (and we're starting to see some confusion) that i feel comfortable removing the master branch entirely.

if your integration breaks, please migrate to the main branch.

thank you

- dominic