Drop docstrings for attributes.

Due to bazelbuild/skydoc#59, Skydoc chokes
on these. They're otherwise unused by any tool. And it's proven hard
to keep them in sync with text documentation. So favour text
documentation for now, until Skydoc comes of age.

haskell/haskell.bzl

@@ -28,26 +28,12 @@ load(":cc.bzl",
 )
 
 _haskell_common_attrs = {
-  "src_strip_prefix": attr.string(
-    mandatory=False,
-    doc="Directory in which module hierarchy starts."
-  ),
-  "srcs": attr.label_list(
-    allow_files=FileType([".hs", ".hsc"]),
-    doc="A list of source files to be built by this rule."
-  ),
-  "copts": attr.string_list(
-    doc="Options to pass to C compiler for any C source files."
-  ),
-  "deps": attr.label_list(
-    doc="haskell_library dependencies"
-  ),
-  "compiler_flags": attr.string_list(
-    doc="Flags to pass to Haskell compiler while compiling this rule's sources."
-  ),
-  "prebuilt_dependencies": attr.string_list(
-    doc="Haskell packages which are magically available such as wired-in packages."
-  ),
+  "src_strip_prefix": attr.string(mandatory=False),
+  "srcs": attr.label_list(allow_files=FileType([".hs", ".hsc"])),
+  "copts": attr.string_list(),
+  "deps": attr.label_list(),
+  "compiler_flags": attr.string_list(),
+  "prebuilt_dependencies": attr.string_list(),
   # XXX Consider making this private. Blocked on
   # https://github.com/bazelbuild/bazel/issues/4366.
   "version": attr.string(
@@ -75,11 +61,9 @@ def _mk_binary_rule(**kwargs):
   return rule(
     _haskell_binary_impl,
     executable = True,
-    attrs = dict(_haskell_common_attrs,
-      main = attr.string(
-        default="Main.main",
-        doc="Main function location.",
-      ),
+    attrs = dict(
+      _haskell_common_attrs,
+      main = attr.string(default="Main.main"),
     ),
     host_fragments = ["cpp"],
     toolchains = ["@io_tweag_rules_haskell//haskell:toolchain"],