Remove unneeded explicit std::string constructors (#2273)

std::string(const char*) implicitly constructs these.  The remaining call sites
requires string literals from C++14.

src/fdcache_entity.cpp

@@ -1035,7 +1035,7 @@ bool FdEntity::SetContentType(const char* path)
         return false;
     }
     AutoLock auto_lock(&fdent_lock);
-    orgmeta["Content-Type"] = S3fsCurl::LookupMimeType(std::string(path));
+    orgmeta["Content-Type"] = S3fsCurl::LookupMimeType(path);
     return true;
 }