Remove unused cache methods

These were added in https://mondrian.corp.google.com/changelist/19077-p9.

CacheManager.setCacheDisabled() has been unused since
https://mondrian.corp.google.com/changelist/20678-p9 (see change to WebView),
so the WebKit cache is always enabled.

BrowserFrame.setCacheDisabled() has been unused since
https://mondrian.corp.google.com/changelist/33679-p9, so the WebCore cache is
always enabled.

BrowserFrame.CacheDisabled() has been unused since
https://mondrian.corp.google.com/changelist/32803-p9 (see change to WebView).

See WebKit change https://android-git.corp.google.com/g/167298.

Change-Id: I68bf334f4a34df1b88d2b5204e50c2eb04db7e69