refactor(material): remove month constants from core

In the past, we accidentally published the month constants
in `@angular/material/core`. These were never intended for
public consumption but were only needed for testing. In v7,
we deprecated the constants (with the assumption that consumers
might have used constants). In version 9, we can remove the constants.

There is no replacement for these variables. Instead, use the zero-based
month index on your own.

BREAKING CHANGE: Deprecated month constants from `@angular/material/core` have been
removed. These have been accidentally published in the past while they
were only intended for internal consumption in tests.

Author: devversion

src/material/core/month-constants.ts

@@ -18,6 +18,3 @@ export * from './option/index';
 export * from './label/label-options';
 export * from './ripple/index';
 export * from './selection/index';
-
-// TODO(devversion): remove this with v8
-export * from './month-constants';