Remove _getChangeUrlAnd methods

This methods obscure the actual calls unnecessarily. As can be seen from
this change, making direct call to restApiHelper is almost the same
amount of code. Most of the implementation was just forwarding
parameters, which is not necessary, since we can construct the "input
object" right away.

Additionally replace send with fetch. Part of the
difference that existed in the past between send() (with parsed: false)
and fetchRawJSON() is that send() used to also check the status of the
response. That's why when we replace send() with fetch(), we set
reportServerError to true.

Moving to fetch removes semantics of send(), where it
sometimes returns parsed response and sometimes plain Response. As
evident from usage in corresponding tests, this used to lead to
confusion. Developers would expect parsed response, where in fact no
parsing is being done.

reportEndpointAsId is likely just a typo that existed previously in the
codebase. It's never read (only set), and the used in the same places
where reportEndpointAsIs makes sense. So we treat it as such.

Similarly reportUrlAsIs is likely a mistake due to this being a
parameter that is used by fetchRawJSON, but not by send(). It would
previously be dropped without being used, with this change we treat it
as reportEndpointAsIs.

The setChangeTopic and setChangeHashtags can return undefined (and could
return 'null' in the past). The return type annotation is wrong here.
There was an issue of undefined result not being processed correctly,
but that has been addressed in previous change

While converting _getDiffComments, i've noticed _getDiffCommentsFetchUrl
is unused. Likely it became unused from edits in the past. Removed.

Google-Bug-Id: b/297849592
Release-Notes: skip
Change-Id: I699a3bb79e3b1e44a70de6b770539708ff156fda

polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts

diff --git a/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts b/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts
index 299d9ac..3314ce4 100644
--- a/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts
+++ b/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts
@@ -135,7 +135,7 @@
   deleteChangeCommitMessage(): Promise<Response> {
     return Promise.resolve(new Response());
   },
-  deleteComment(): Promise<CommentInfo> {
+  deleteComment(): Promise<CommentInfo | undefined> {
     throw new Error('deleteComment() not implemented by RestApiMock.');
   },
   deleteDiffDraft(): Promise<Response> {
@@ -165,7 +165,7 @@
   deleteWatchedProjects(): Promise<Response> {
     return Promise.resolve(new Response());
   },
-  executeChangeAction(): Promise<Response | undefined> {
+  executeChangeAction(): Promise<Response> {
     return Promise.resolve(new Response());
   },
   finalize(): void {},