Revert "Track update count of IMMS#mMethodMap"

This reverts commit e073306d [1].

Reason for revert:
This field is not so useful any more, especially after we started
offloading slow disk read operations to a worker thread without
requiring ImfLock [2][3].
Let's simplify this unused field in favor of simplicity.

 [1]: I05850578cbf2e6adfea6c7e9b1fb9ec4e9d5e2c9
 [2]: I263cd49dd4d64b64136acc3dad469f83a862ce97
      db19711e
 [3]: Icc84fe9d0d3b5842b4d5ee51b12b0d3cd10a1d24
      81a9a1ba

Bug: 32343335
Bug: 340221861
Bug: 343601565
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I424b2bab3921d0354638df40877b94449e5b7af8