[CI] Remove workflow_dispatch way for image build (#5742)

There is some problem for workflow_dispatch way for image build. Let's
remove it first to make CI happy. I'll add it back once it's well
tested.

- vLLM version: v0.13.0
- vLLM main:
2f4e6548ef

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

.github/workflows/_schedule_image_build.yaml

@@ -148,7 +148,6 @@ jobs:
           tags: |
             type=pep440,pattern={{raw}},suffix=${{ env.SUFFIX }}
             type=schedule,pattern=main,suffix=${{ env.SUFFIX }}
-            type=raw,value=${{ inputs.workflow_dispatch_tag }},event=workflow_dispatch,suffix=${{ env.SUFFIX }}
           flavor:
             latest=false
 

.github/workflows/schedule_image_build_and_push.yaml

@@ -21,7 +21,7 @@ on:
       tag:
         description: 'Docker tag for build results'
         type: string
-        default: manual
+        default: main
         required: true
 
 jobs: