Revert "Pin system/core to avoid adb dependency on libandroidfw."

This reverts commit a5505518.

Reason for revert: CL that removes the dependency is in:
https://android-review.googlesource.com/c/platform/system/core/+/740521

Change-Id: If816e5c7b0184c7c6c4753bcc1f8e2a2bcdc1893

default.xml

@@ -126,7 +126,7 @@
   <project path="prebuilts/sdk" name="platform/prebuilts/sdk" groups="pdk" clone-depth="1" />
   <project path="prebuilts/tools" name="platform/prebuilts/tools" groups="pdk,tools" clone-depth="1" />
   <project path="sdk" name="platform/sdk" groups="pdk-cw-fs,pdk-fs" />
-  <project path="system/core" name="platform/system/core" groups="pdk" revision="de39afd1e0274c3005d60884912bf825ed7d5c00"/>
+  <project path="system/core" name="platform/system/core" groups="pdk" />
   <project path="system/extras" name="platform/system/extras" groups="pdk" />
   <project path="system/keymaster" name="platform/system/keymaster" groups="pdk" />
   <project path="system/netd" name="platform/system/netd" groups="pdk" />