fix(ios): remove hardcoded X-Requested-With header (#287)

no other platform sets this header and it should be up to the user if it's desired to 'disguise' the HTTP call as an XMLHttpRequest
diff --git a/src/ios/CDVFileTransfer.m b/src/ios/CDVFileTransfer.m
index 5d556b0..66cb9a1 100644
--- a/src/ios/CDVFileTransfer.m
+++ b/src/ios/CDVFileTransfer.m
@@ -102,8 +102,6 @@
 
 - (void)applyRequestHeaders:(NSDictionary*)headers toRequest:(NSMutableURLRequest*)req
 {
-    [req setValue:@"XMLHttpRequest" forHTTPHeaderField:@"X-Requested-With"];
-
     for (NSString* headerName in headers) {
         id value = [headers objectForKey:headerName];
         if (!value || (value == [NSNull null])) {