Update Dockerfiles for python artifacts to use latest git version

Due to a recent change (Feb 2018) in Github's policy to discontinue
support for weak cryptographic standards, older git versions stopped
working with Github repositories. The Dockerfiles for building python
artifacts currently uses git 2.0.5, that fails with an error `tlsv1
alert protocol version`.

The CentOS image by the latest source Dockerfile, `manylinux` comes
with the latest version of git as well, removing the need to manually
build git from source.

tools/dockerfile/grpc_artifact_python_manylinux_x64/Dockerfile

@@ -19,19 +19,6 @@ FROM quay.io/pypa/manylinux1_x86_64
 # Update the package manager
 RUN yum update -y
 
-#############################################################
-# Update Git to allow cloning submodules with --reference arg
-RUN yum remove -y git
-RUN yum install -y curl-devel expat-devel gettext-devel linux-headers openssl-devel zlib-devel gcc
-RUN cd /usr/src && \
-  curl -O -L https://kernel.org/pub/software/scm/git/git-2.0.5.tar.gz && \
-  tar xzf git-2.0.5.tar.gz
-RUN cd /usr/src/git-2.0.5 && \
-  make prefix=/usr/local/git all && \
-  make prefix=/usr/local/git install
-ENV PATH /usr/local/git/bin:$PATH
-RUN source /etc/bashrc
-
 ###################################
 # Install Python build requirements
 RUN /opt/python/cp27-cp27m/bin/pip install cython

tools/dockerfile/grpc_artifact_python_manylinux_x86/Dockerfile

@@ -19,19 +19,6 @@ FROM quay.io/pypa/manylinux1_i686
 # Update the package manager
 RUN yum update -y
 
-#############################################################
-# Update Git to allow cloning submodules with --reference arg
-RUN yum remove -y git
-RUN yum install -y curl-devel expat-devel gettext-devel linux-headers openssl-devel zlib-devel gcc
-RUN cd /usr/src && \
-  curl -O -L https://kernel.org/pub/software/scm/git/git-2.0.5.tar.gz && \
-  tar xzf git-2.0.5.tar.gz
-RUN cd /usr/src/git-2.0.5 && \
-  make prefix=/usr/local/git all && \
-  make prefix=/usr/local/git install
-ENV PATH /usr/local/git/bin:$PATH
-RUN source /etc/bashrc
-
 ###################################
 # Install Python build requirements
 RUN /opt/python/cp27-cp27m/bin/pip install cython