vold: remove checkEncryption

The testing-only "checkEncryption" command has been replaced with
VtsKernelEncryptionTest [1].  In particular, the metadata encryption
documentation [2] no longer mentions checkEncryption but rather
recommends running VtsKernelEncryptionTest.  Also, checkEncryption
hasn't really been maintained, and it has some known bugs; it's unclear
that many people ever really used it.

So, just remove it.

[1] https://android.googlesource.com/platform/test/vts-testcase/kernel/+/refs/heads/master/encryption/
[2] https://source.android.com/security/encryption/metadata#tests

Bug: 155037012
Change-Id: I258829c60768fd11aafdf2faad956cecc1ae9826