external.md: remove unjustified recommendation
There is no real advantage in using build_file_content over using
a build_file argument pointing to a meterialized BUILD file. On the
contrary, meaningful history and reuse among several projects having
the same dependency is easier if the imported BUILD file exists
as actual file.
@@ -269,10 +269,6 @@ Do not use `bind()`. See "[Consider removing
bind](https://github.com/bazelbuild/bazel/issues/1952)" for a long discussion of its issues and
alternatives.
-### Custom BUILD files
-
-When using a `new_` repository rule, prefer to specify `build_file_content`, not `build_file`.
-
### Repository rules
A repository rule should generally be responsible for:
