Reverts "[ci] Temporarily allow-warnings in podspec_check_command.dart (flutter#6416)" (flutter#6419)

Reverts: flutter/packages#6416
Initiated by: ditman
Reason for reverting: Failures happen (later) in other plugins (more updates in the related issue)

flutter#145866
Original PR Author: ditman

Reviewed By: {jmagman}

This change reverts the following previous change:
Adds `--allow-warnings` to podspec_check_command.dart so stuff can keep rolling.

**To be removed soon!**

Author: auto-submit[bot]

script/tool/lib/src/podspec_check_command.dart

@@ -151,7 +151,6 @@ class PodspecCheckCommand extends PackageLoopingCommand {
       podspecPath,
       '--configuration=Debug', // Release targets unsupported arm64 simulators. Use Debug to only build against targeted x86_64 simulator devices.
       '--skip-tests',
-      '--allow-warnings',
       '--use-modular-headers', // Flutter sets use_modular_headers! in its templates.
       if (libraryLint) '--use-libraries'
     ];

script/tool/test/podspec_check_command_test.dart

@@ -156,7 +156,6 @@ void main() {
                     .path,
                 '--configuration=Debug',
                 '--skip-tests',
-                '--allow-warnings',
                 '--use-modular-headers',
                 '--use-libraries'
               ],
@@ -172,7 +171,6 @@ void main() {
                     .path,
                 '--configuration=Debug',
                 '--skip-tests',
-                '--allow-warnings',
                 '--use-modular-headers',
               ],
               packagesDir.path),
@@ -214,7 +212,6 @@ void main() {
                     .path,
                 '--configuration=Debug',
                 '--skip-tests',
-                '--allow-warnings',
                 '--use-modular-headers',
                 '--use-libraries'
               ],
@@ -230,7 +227,6 @@ void main() {
                     .path,
                 '--configuration=Debug',
                 '--skip-tests',
-                '--allow-warnings',
                 '--use-modular-headers',
               ],
               packagesDir.path),