libstore: remove FileTransferRequest::mimeType

it's only used by HttpBinaryCacheStore, and even there used in only on
place. this one place can set the header explicitly, which it now does

Change-Id: Id89228150669e25e7f59a3d6bd939e46059ce29e

src/libstore/filetransfer.cc

@@ -97,8 +97,6 @@ struct curlFileTransfer : public FileTransfer
             requestHeaders = curl_slist_append(requestHeaders, "Accept-Encoding: zstd, br, gzip, deflate, bzip2, xz");
             if (!request.expectedETag.empty())
                 requestHeaders = curl_slist_append(requestHeaders, ("If-None-Match: " + request.expectedETag).c_str());
-            if (!request.mimeType.empty())
-                requestHeaders = curl_slist_append(requestHeaders, ("Content-Type: " + request.mimeType).c_str());
             for (auto it = request.headers.begin(); it != request.headers.end(); ++it){
                 requestHeaders = curl_slist_append(requestHeaders, fmt("%s: %s", it->first, it->second).c_str());
             }

src/libstore/filetransfer.hh

@@ -63,7 +63,6 @@ struct FileTransferRequest
     unsigned int baseRetryTimeMs = 250;
     ActivityId parentAct;
     std::optional<std::string> data;
-    std::string mimeType;
 
     FileTransferRequest(std::string_view uri)
         : uri(uri), parentAct(getCurActivity()) { }

src/libstore/http-binary-cache-store.cc

@@ -134,7 +134,7 @@ protected:
     {
         FileTransferRequest req{makeURI(path)};
         req.data = StreamToSourceAdapter(istream).drain();
-        req.mimeType = mimeType;
+        req.headers = {{"Content-Type", mimeType}};
         try {
             getFileTransfer()->transfer(req);
         } catch (FileTransferError & e) {