e2fsck: drop -f when TARGET_USES_MKE2FS is enabled

- Drop forced (-f) e2fsck check when the product has
  enabled new ext4 generation by setting TARGET_USES_MKE2FS.
- The new generation tool is supposed to give better stability,
  thus justifying dropping -f.
- This should help reducing boot-up time as full check (-f) can
  increase boot-up time significantly depending on amount of data.

bug: 32246772
Test: many reboots
Change-Id: I631525bf7504bbfb025e170c8d24ad9d3ef3532e