Remove the drain argument from readFile

Now it is always `drain` (see previous commit).

src/libutil/util.hh

@@ -103,7 +103,7 @@ unsigned char getFileType(const Path & path);
 
 /* Read the contents of a file into a string. */
 string readFile(int fd);
-string readFile(const Path & path, bool drain = false);
+string readFile(const Path & path);
 void readFile(const Path & path, Sink & sink);
 
 /* Write a string to a file. */