Remove `gdbserver` from `PRODUCT_PACKAGES_DEBUG` in `base_system.mk`.

The `gdbserver` binaries have been removed from Android sources
(https://android-review.googlesource.com/c/platform/prebuilts/misc/+/1748908/).

Test: m
Change-Id: I8e3001508360906f3fe86830b81fb94b01a5ac74