csv-filter: remove (#341)

We don't have enough maintenance resource for it. Someone who wants to
maintain this can restart from

https://github.com/ruby/csv/blob/7eea19a639d6c03829b337e5c916804d8e1c10cc/bin/csv-filter
.

See also: #340 (comment)

Author: kou

bin/csv-filter

@@ -38,7 +38,6 @@ Gem::Specification.new do |spec|
     end
   end
   spec.files = files
-  spec.executables = ["csv-filter"]
   spec.rdoc_options.concat(["--main", "README.md"])
   rdoc_files = [
     "LICENSE.txt",