Revert "logd: logcat --clear respect pruneMargin"

This reverts commit 0878a7c1.

This is a partial revert of the above commit.  It simply removes
pruneMargin from these calculations since it is going away.

Bug: 149340579
Test: --clear works well
Change-Id: I1763be56fae7052058a800fad3b295c73cdcadf6