Dismantle SystemJapanesePhysicalTyping CrOS 1P IME shared lib flag.

This flag is obsolete. It's always-ON, and ChromeOS 1P
IME shared lib (in internal repo) no longer reads it.

Must NOT be submitted until internal http://cl/813191244
has landed in a Chromium's CrOS LKGM. That's now the case
with http://cl/813191244 landed @ R143-16443.0.0 [2] while
LKGM [2] at ToT is now @ 16447.0.0-1072370.

[1] https://crrev.com/i/8633959
[2] https://crsrc.org/c/chromeos/CHROMEOS_LKGM

Bug: b:444378290
Change-Id: Ia671cc8cd238ba06de7354e3fd5c5377a27f44ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6998850
Reviewed-by: John Palmer <jopalmer@chromium.org>
Commit-Queue: Bao-Duy Tran <tranbaoduy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1528743}