Remove a few TODO(GYP)s that look obsolete.

It looks like nobody needed QNX support, so remove TODOs about adding
this.

Likewise, gyp used to support use_system_icu, which when set added
`icu-config --ldflags` to ldflags and `icu-config --ldflags-libsonly`
to libs. Apparently nobody missed it, so remove the TODO about it.
It looks like distros now use
https://cs.chromium.org/chromium/src/build/linux/unbundle/icu.gn
for icu instead.

Finally remove a comment referring to things being confusing in
the gyp build.

No behavior change.

Bug: none
Change-Id: Ibefd0998dc65ab1bcd88e3fa95a160debbd65149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1801855


Reviewed-by: Jungshik Shin <jshin@chromium.org>