Remove obsolete ".txt" extensions for AsciiDoc files

Since we no longer have any AsciiDoc files that end in ".txt", don't
modify them with .gitattributes or ignore them with .gitignore.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitattributes

@@ -7,7 +7,6 @@
 *.py text eol=lf diff=python
 *.bat text eol=crlf
 CODE_OF_CONDUCT.md -whitespace
-/Documentation/**/*.txt text eol=lf
 /Documentation/**/*.adoc text eol=lf
 /command-list.txt text eol=lf
 /GIT-VERSION-GEN text eol=lf

Documentation/.gitignore

@@ -6,14 +6,10 @@
 *.pdf
 git.info
 gitman.info
-howto-index.txt
 howto-index.adoc
 doc.dep
-cmds-*.txt
 cmds-*.adoc
-mergetools-*.txt
 mergetools-*.adoc
-SubmittingPatches.txt
 SubmittingPatches.adoc
 tmp-doc-diff/
 tmp-meson-diff/