Removing a system app isn't WebView-specific, but it's occasionally necessary for WebView development.
We have an automated script to remove the preinstalled WebView from the device, but if this script does not work or you need to remove a different system app, you can manually run the following adb commands. This uses "com.google.android.webview" as an example, but change the package name as necessary for your case.
# Uninstall updates. Repeat "adb uninstall" until it fails with the
# "DELETE_FAILED_INTERNAL_ERROR" error message to make sure you've removed all
# the updates.
$ adb uninstall com.google.android.webview
Success
$ adb uninstall com.google.android.webview
Failure [DELETE_FAILED_INTERNAL_ERROR]
# Figure out the path of the system app. This varies depending on OS level.
$ adb shell pm path com.google.android.webview
package:/system/app/WebViewGoogle/WebViewGoogle.apk
$ adb root
# This is necessary for M and up:
$ adb disable-verity
$ adb reboot
# wait for device to reboot...
$ adb root
# For all OS versions:
# Mount the system partition as read-write and 'rm' the path we found before.
$ adb remount
$ adb shell stop
$ adb shell rm -rf /system/app/WebViewGoogle/WebViewGoogle.apk
$ adb shell start