Switch to html-textview from JCenter

.gitmodules

@@ -2,10 +2,6 @@
 	path = extern/spongycastle
 	url = https://github.com/open-keychain/spongycastle.git
 	ignore = dirty
-[submodule "extern/html-textview"]
-	path = extern/html-textview
-	url = https://github.com/open-keychain/html-textview.git
-	ignore = dirty
 [submodule "extern/openpgp-api-lib"]
 	path = extern/openpgp-api-lib
 	url = https://github.com/open-keychain/openpgp-api-lib.git