Move 'remount' back into 'adbd_system_api'

It was inlined in base_system.mk during the adbd apex bring up phase,
because adbd apex was thought to have broken "required:" dependency.
Now that we have 'adb_system_api' to track the dependencies of adbd, we
can move 'remount' into the phony module.

Bug: 241688845
Test: m adbd_system_api w/ user & userdebug build
Change-Id: I5d7c66c517d5e7022601cc4705a447d5a4ba1be4

target/product/base_system.mk

@@ -378,7 +378,6 @@ PRODUCT_PACKAGES_DEBUG := \
     procrank \
     profcollectd \
     profcollectctl \
-    remount \
     servicedispatcher \
     showmap \
     sqlite3 \