Revert "[ios] Update Firebase library to 6.16"

This reverts commit ac791f4c.

Reason for revert: Compile build fails.

Original change's description:
> [ios] Update Firebase library to 6.16
> 
> This brings in the new library files as a CIPD package and update
> DEPS to refer to this updated library.
> 
> Fixed: 1049042
> Change-Id: I874f1a148bd7c47a4756d25f5edb590a39709b0d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042215
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#739162}

TBR=eugenebut@chromium.org,pkl@chromium.org,thegreenfrog@chromium.org

Change-Id: I225f141037073ac8d7926bd2c96825b94b74a0da
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2043081Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739248}

DEPS

@@ -511,7 +511,7 @@ deps = {
       'packages': [
         {
           'package': 'chromium/third_party/firebase_ios',
-          'version': 'LF-Sg4wxHfilYCXiSZoj0gcLub1ueZX-q8H-xAVUrKQC',
+          'version': 'QmLncusdxHhHwXryoNEHHUGacx1Tg5ij449uwZL2dpAC',
         },
       ],
       'condition': 'checkout_ios',

ios/third_party/firebase/README.chromium

 Name: Firebase SDK
 Short Name: Firebase
 URL: https://firebase.google.com/terms/
-Version: 6.16.0
+Version: 5.18.1
 License: Custom License
 License File: LICENSE