tools: Docker: remove libpython2.7

libpython2.7 was added to the container to allow running GDB built
with Python 2.7 support and distributed as part of the cross-compiler
toolchain.

Now that we have a new release of GDB which works with Python 3.x,
the GDB shipped with the cross-compiler is no longer used. Removing
libpython2.7 should reduce the image size.

This reverts commit be0372b1db.

tools/docker/Dockerfile

@@ -2,7 +2,6 @@ FROM ubuntu:20.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 
-# We need libpython2.7 due to GDB tools
 RUN : \
   && apt-get update \
   && apt-get install -y \
@@ -20,7 +19,6 @@ RUN : \
     libbsd-dev \
     libffi-dev \
     libncurses-dev \
-    libpython2.7 \
     libusb-1.0-0-dev \
     make \
     ninja-build \