Revert release_version "fix" for mbed 2.

The online compiler requires mbed os 2 support. Now release_version has
been partially removed from the tools, we just need to return all targets
when any version is given. This doesn't really make sense, as now we just
claim to support all targets in all release_versions. However, it allows the
online compiler to find targets for mbed 2 projects, which we still need
to support online.

We need to come up with a mechanism for maintaining release_version
support in the online tools, this unfortunate patch is just to get
things working for the Mbed 6 release.

tools/build_api.py

@@ -416,15 +416,6 @@ def get_mbed_official_release(version, profile=None):
     version - The version string. Should be a string contained within
               RELEASE_VERSIONS
     """
-
-    # we ignore version for Mbed 6 as all targets in targets.json file are being supported
-    # if someone passes 2, we return empty tuple, if 5, we keep the behavior the same as 
-    # release version is deprecated and all targets are being supported that are present
-    # in targets.json file
-
-    if version == '2':
-        return tuple(tuple([]))
-
     mbed_official_release = (
         tuple(
             tuple(
@@ -433,11 +424,18 @@ def get_mbed_official_release(version, profile=None):
                     tuple(transform_release_toolchains(
                         TARGET_MAP[target], version))
                 ]
-            ) for target in TARGET_NAMES \
-                if not profile or profile in TARGET_MAP[target].supported_application_profiles
+            ) for target in TARGET_NAMES
+            if (hasattr(TARGET_MAP[target], 'release_versions')
+                and version in TARGET_MAP[target].release_versions)
         )
     )
 
+    for target in mbed_official_release:
+        is_official, reason = is_official_target(target[0], version)
+
+        if not is_official:
+            raise InvalidReleaseTargetException(reason)
+
     return mbed_official_release
 
 def target_supports_toolchain(target, toolchain_name):