[infra] Remove linux SDK builder test matrix entry.

Both the production and try builders for the Linux SDK have been
migrated to the new recipe that doesn't use the test matrix.

The tools/bots/dart_sdk.py script is is no longer use and is removed.

Bug: b/253079985
Change-Id: I85a122b6165858b3dd43e7a1baad1dcd23f32ecb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265341
Reviewed-by: Alexander Thomas <[email protected]>
Commit-Queue: Jonas Termansen <[email protected]>

tools/bots/test_matrix.json

@@ -3149,51 +3149,6 @@
         }
       ]
     },
-    {
-      "builders": [
-        "dart-sdk-linux"
-      ],
-      "meta": {
-        "description": "This configuration is used by the linux sdk builder. The difference between this configuration and the other sdk builder configurations is that this one also generates the api docs."
-      },
-      "steps": [
-        {
-          "name": "build dart (clang builds)",
-          "script": "tools/build.py",
-          "arguments": [
-            "--arch=ia32,x64,arm,arm64",
-            "--mode=release",
-            "--check-clean",
-            "create_sdk"
-          ]
-        },
-        {
-          "name": "build dart (gcc builds)",
-          "script": "tools/build.py",
-          "arguments": [
-            "--no-clang",
-            "--arch=riscv64",
-            "--mode=release",
-            "--check-clean",
-            "create_sdk"
-          ]
-        },
-        {
-          "name": "upload sdk",
-          "script": "tools/bots/dart_sdk.py",
-          "arguments": [
-            "--arch=ia32,x64,arm,arm64,riscv64"
-          ]
-        },
-        {
-          "name": "build api docs",
-          "script": "tools/bots/dart_sdk.py",
-          "arguments": [
-            "api_docs"
-          ]
-        }
-      ]
-    },
     {
       "builders": [
         "debianpackage-linux"