libarchive: Remove CMake-specific README

We will now manage the libarchive source tree updates using
the `Utilities/Scripts/update-libarchive.bash` script.  Drop
the README that covered the old method.

Utilities/cmlibarchive/README-CMake.txt

-The Utilities/cmlibarchive directory contains a reduced distribution
-of the libarchive source tree with only the library source code and
-CMake build system.  It is not a submodule; the actual content is part
-of our source tree and changes can be made and committed directly.
-
-We update from upstream using Git's "subtree" merge strategy.  A
-special branch contains commits of upstream libarchive snapshots and
-nothing else.  No Git ref points explicitly to the head of this
-branch, but it is merged into our history.
-
-Update libarchive from upstream as follows.  Create a local branch to
-explicitly reference the upstream snapshot branch head:
-
- git branch libarchive-upstream 1a8c7bc2
-
-Use a temporary directory to checkout the branch:
-
- mkdir libarchive-tmp
- cd libarchive-tmp
- git init
- git pull .. libarchive-upstream
- rm -rf *
-
-Now place the (reduced) libarchive content in this directory.  See
-instructions shown by
-
- git log 1a8c7bc2
-
-for help extracting the content from the upstream svn repo.  Then run
-the following commands to commit the new version.  Substitute the
-appropriate date and version number:
-
- git add --all
-
- GIT_AUTHOR_NAME='LibArchive Upstream' \
- GIT_AUTHOR_EMAIL='libarchive-discuss@googlegroups.com' \
- GIT_AUTHOR_DATE='Wed Oct 21 01:47:34 2015 -0700' \
- git commit -m 'libarchive 3.1.2-601-g3bfe5f1 (reduced)' &&
- git commit --amend
-
-Edit the commit message to describe the procedure used to obtain the
-content.  Then push the changes back up to the main local repository:
-
- git push .. HEAD:libarchive-upstream
- cd ..
- rm -rf libarchive-tmp
-
-Create a topic in the main repository on which to perform the update:
-
- git checkout -b update-libarchive master
-
-Merge the libarchive-upstream branch as a subtree:
-
- git merge -s recursive -X subtree=Utilities/cmlibarchive \
-           libarchive-upstream
-
-If there are conflicts, resolve them and commit.  Build and test the
-tree.  Commit any additional changes needed to succeed.
-
-Finally, run
-
- git rev-parse --short=8 libarchive-upstream
-
-to get the commit from which the libarchive-upstream branch must be started
-on the next update.  Edit the "git branch libarchive-upstream" line above to
-record it, and commit this file.