Revert "Add automatic default value for char-type field" am: ac1cb3eb26 am: e7f67dccd7

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/tools/aidl/+/16381822

Change-Id: I8a6cd4f57324b1c053f7f2044c6a3b624ca9fe47