Remove multiapi_combiner tool and clean up multiapi references

Co-authored-by: msyyc <[email protected]>

Author: Copilot

eng/swagger_to_sdk_config_v4.json

@@ -7,7 +7,6 @@
       "python": "",
       "python-mode": "update",
       "sdkrel:python-sdks-folder": "./sdk/.",
-      "multiapi": "",
       "keep-version-file" :"",
       "no-async": ""
     },

eng/tools/azure-sdk-tools/changelog_generics.md

@@ -41,10 +41,7 @@ In summary, some modules were incorrectly visible/importable and have been renam
 
 Last but not least, HTTP connection pooling is now enabled by default. You should always use a client as a context manager, or call close(), or use no more than one client per process.
 
-# Autorest v3 to v4, if the package is multiapi
-
-
-## General Breaking changes
+# Breaking changes in generated packages
 
 This version uses a next-generation code generator that *might* introduce breaking changes if you were importing from the v20xx_yy_zz API folders.
 In summary, some modules were incorrectly visible/importable and have been renamed. This fixed several issues caused by usage of classes that were not supposed to be used in the first place.