[ios] Remove *_blink_app_ext variants of toolchains

It looks like those toolchains are no longer used. They were added
and used in https://crrev.com/c/5332159 and then the last usage was
removed in https://crrev.com/c/5499634.

Additionally, with https://crrev.com/c/7045128 the *_app_ext variant
do not override `use_partition_alloc` if `use_blink = true` and the
toolchains where only used when `use_blink` was globally set to true
so the *_blink_app_ext and *_app_ext variant were identical.

Cq-Include-Trybots: luci.chromium.try:ios-blink-rel-fyi
Change-Id: I3a66c1288accfd15ae01427bbb8cb644dfc50739
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7045497
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530716}
NOKEYCHECK=True
GitOrigin-RevId: 253b54403153ac1142cc53e78236716fecc776de