Remove unused code DataSource::getDrmInfo

The method in question was added in I4b9ef19 with DRMExtractor.
The only use case of getDrmInfo(), in DRMExtractor, has been removed
with I4dd3f07ca.

Test: build. post commit media cts test.
Bug: 65851881
Change-Id: Ieb4d9d36c74efcfbfe690bcd09e1cd7e08ee3f1a