jsoncpp: Drop outdated CMake-specific README

Since commit ee68d3eb8c (jsoncpp: Add script to update from upstream,
2017-08-28, v3.10.0-rc1~199^2~8) we use a script to maintain the jsoncpp
vendor branch.  Drop our readme that documented the old approach.

Utilities/cmjsoncpp/README-CMake.txt

@@ -1,66 +0,0 @@
-The Utilities/cmjsoncpp directory contains a reduced distribution
-of the jsoncpp 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 jsoncpp snapshots and
-nothing else.  No Git ref points explicitly to the head of this
-branch, but it is merged into our history.
-
-Update jsoncpp from upstream as follows.  Create a local branch to
-explicitly reference the upstream snapshot branch head:
-
- git branch jsoncpp-upstream 53f6ccb0
-
-Use a temporary directory to checkout the branch:
-
- mkdir jsoncpp-tmp
- cd jsoncpp-tmp
- git init
- git pull .. jsoncpp-upstream
- rm -rf *
-
-Now place the (reduced) jsoncpp content in this directory.  See
-instructions shown by
-
- git log 53f6ccb0
-
-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='JsonCpp Upstream' \
- GIT_AUTHOR_EMAIL='[email protected]' \
- GIT_AUTHOR_DATE='Thu Nov 20 08:45:58 2014 -0600' \
- git commit -m 'JsonCpp 1.0.0 (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:jsoncpp-upstream
- cd ..
- rm -rf jsoncpp-tmp
-
-Create a topic in the main repository on which to perform the update:
-
- git checkout -b update-jsoncpp master
-
-Merge the jsoncpp-upstream branch as a subtree:
-
- git merge -s recursive -X subtree=Utilities/cmjsoncpp \
-           jsoncpp-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 jsoncpp-upstream
-
-to get the commit from which the jsoncpp-upstream branch must be started
-on the next update.  Edit the "git branch jsoncpp-upstream" line above to
-record it, and commit this file.