[eng] deprecate "multiapi" - Remove multiapi functionality from eng folder (#42866)

* Initial plan

* Remove multiapi_combiner tool and clean up multiapi references

Co-authored-by: msyyc <70930885+msyyc@users.noreply.github.com>

* Clean up unused import after multiapi removal

Co-authored-by: msyyc <70930885+msyyc@users.noreply.github.com>

* Update sdk_generator.py

* Improve logging message for SDK folder validation

* format

* update

* format

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: msyyc <70930885+msyyc@users.noreply.github.com>
Co-authored-by: Yuchao Yan <yuchaoyan@microsoft.com>
Co-authored-by: ChenxiJiang333 <119990644+ChenxiJiang333@users.noreply.github.com>

Author: 3 people

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.

eng/tools/azure-sdk-tools/packaging_tools/package_utils.py

@@ -77,13 +77,12 @@ def change_log_generate(
     *,
     last_stable_release: Optional[str] = None,
     prefolder: Optional[str] = None,
-    is_multiapi: bool = False,
 ):
     if not last_version:
         return "### Other Changes\n\n  - Initial version"
 
     # try new changelog tool
-    if prefolder and not is_multiapi:
+    if prefolder:
         try:
             tox_cache_path = Path(prefolder, package_name, ".tox")
             if tox_cache_path.exists():