Remove no longer needed getErrorString(...)

The strings are now obtained by the manager classes, such as
FingerprintManager and FaceManager. BiometricService just passes down
the error codes.

Bug: 141025588
Test: atest BiometricServiceTest
Test: the errors are shown correctly in BiometricPromptDemo
Change-Id: I19711c81dc86eae5e0c5d12eaea9a2a9a5bd1f2c