Revert "sources(curl): disable curl --parallel by default"

This reverts commit 9bef57d5a6.

sources/org.osbuild.curl

@@ -110,12 +110,6 @@ def curl_has_parallel_downloads():
     Return true if curl has all the support we needed for parallel downloading
     (this include --write-out "%{json}" too
     """
-    # Re-enable this once the failures in
-    # https://github.com/osbuild/osbuild-composer/pull/4247
-    # are understood
-    if os.getenv("OSBUILD_SOURCES_CURL_USE_PARALLEL") not in ["1", "yes", "true"]:
-        return False
-
     output = subprocess.check_output(["curl", "--version"], universal_newlines=True)
     first_line = output.split("\n", maxsplit=1)[0]
     m = re.match(r'^curl (\d+\.\d+\.\d+)', first_line)