Drop obsolete downstream patches (#6208)

Over time, as upstream Git absorbs fixes and features that originated in
or were carried by Git for Windows, downstream patches accumulate that
are no longer needed. The steady stream of merged PRs makes this
virtually inevitable. This PR collects fixup! commits to drop three such
patches during the next autosquash rebase.

The HTTP/2 workaround in `t5551` was a temporary fix for a libcurl
v8.10.0 regression on macOS CI runners. The faulty libcurl has long been
superseded by fixed versions, making it unnecessary.

The `unix-socket: avoid leak when initialization fails` patch changed
`return -1` to `goto fail` in `unix_stream_connect()` so cleanup would
run when `unix_sockaddr_init()` failed. Upstream fixed the same leak
more surgically in c5fe29f696 (unix-socket: fix memory leak when
chdir(3p) fails, 2025-01-30) by having `unix_sockaddr_init()` call
`FREE_AND_NULL(ctx->orig_dir)` before returning, making the downstream
caller-side fix redundant.

The `revision: create mark_trees_uninteresting_dense()` commit was a
preparatory patch for the path-walk API. That API has since been
upstreamed, and this commit became empty during the merging-rebase
because its changes were already in the new base.

t/t5551-http-fetch-smart.sh

@@ -413,15 +413,7 @@ test_expect_success CMDLINE_LIMIT \
 	)
 '
 
-# This is a temporary work-around for libcurl v8.10.0 on the macos-* runners;
-# see https://github.com/git-for-windows/git/issues/5159 for full details
-test_lazy_prereq UNBROKEN_HTTP2 '
-	test "$HTTP_PROTO" = HTTP/2 &&
-	test -z "$(brew info -q curl 2>/dev/null |
-		sed -n "/^Installed/{N;s/.*8\\.10\\.0.*/BROKEN HTTP2/p;}")"
-'
-
-test_expect_success UNBROKEN_HTTP2 'large fetch-pack requests can be sent using chunked encoding' '
+test_expect_success 'large fetch-pack requests can be sent using chunked encoding' '
 	GIT_TRACE_CURL=true git -c http.postbuffer=65536 \
 		clone --bare "$HTTPD_URL/smart/repo.git" split.git 2>err &&
 	{