Remove signature paramter in fs-verity setup

Now that all the callers pass `null` signature, delete the code in the
non-null path.

Test: atest ChecksumsTest ApkVerityTest UpdatableSystemFontTest
      CtsAppSecurityHostTestCases:android.appsecurity.cts.ApkVerityInstallTest
Bug: 249158715
Change-Id: I865703721a0a094112380910d6ff2c9b22e56042