wear_ui: Remove kMaxCols/kMaxRows/visible_text_rows/menu_headers_.

We've deprecated kMaxCols/kMaxRows in ScreenRecoveryUI since commit
aa0d6afb. They unnecessarily limit the
screen size to 96x96 characters.

Also remove the recomputations of text_cols, visible_text_rows (which is
the same as text_rows_). Remove the unintentional hiding of menu_headers_.

Test: mmma bootable/recovery
Change-Id: I7d1526b651943312d62d52cd200414b42bf9b12a