Remove webviewchromium from PRODUCT_BOOT_JARS for real.

The WebView Java code is now loaded by WebViewFactory from an APK
when an app needs it, so remove the old jar from the boot classpath.
The previous change removed the webview from core_minimal.mk but
not from core_base.mk, which is what this change addresses.

Change-Id: I6f4bcd427a7e65f3a20372c3c177e9342a953290