xds: Get rid of xDS v2 dependencies

[sergiitk]

xDS v2 support was dropped about a year ago, but the xds package still had a few xDS v2 usages. This PR:

• Removes all leftover usages of xDS v2 classes in gprc-xds: 3438f2d, 83e6f0d
• Removes all imported xDS v2 protos and their leaf dependencies: 9754d0b, c24b798
• Removes xDS v2 generated services: 3d83fe3
• Makes minor improvements to the xds import script output: 7997d32

Before

# Imported 154 protos.
❯ find . -iname "*xds*.jar" -exec du -h {} \; | col -x
13M     ./build/libs/grpc-xds-1.63.0-SNAPSHOT-original.jar
6.1M    ./build/libs/grpc-xds-1.63.0-SNAPSHOT-sources.jar
388K    ./build/libs/grpc-xds-1.63.0-SNAPSHOT-javadoc.jar
14M     ./build/libs/grpc-xds-1.63.0-SNAPSHOT.jar

After

# Imported 86 protos.
❯ find . -iname "*xds*.jar" -exec du -h {} \; | col -x
9.1M    ./build/libs/grpc-xds-1.63.0-SNAPSHOT-original.jar
4.1M    ./build/libs/grpc-xds-1.63.0-SNAPSHOT-sources.jar
388K    ./build/libs/grpc-xds-1.63.0-SNAPSHOT-javadoc.jar
9.1M    ./build/libs/grpc-xds-1.63.0-SNAPSHOT.jar

Reduction:

• Number of protos: 44%
• Jar size: 35%