Remove sc_adb_get_serialno()

The device serial is now retrieved from `adb devices -l`, `adb
get-serialno` is not called anymore.

PR #3005 <https://github.com/Genymobile/scrcpy/pull/3005>

app/src/adb/adb.h

@@ -86,14 +86,6 @@ char *
 sc_adb_getprop(struct sc_intr *intr, const char *serial, const char *prop,
                unsigned flags);
 
-/**
- * Execute `adb get-serialno`
- *
- * Return the result, to be freed by the caller, or NULL on error.
- */
-char *
-sc_adb_get_serialno(struct sc_intr *intr, unsigned flags);
-
 /**
  * Attempt to retrieve the device IP
  *