go/toolchains.rst: Remove reference to deprecated go_rule (bazelbuild…

…#3486)

The definition of the go_rule function in now prints the following
message:

    The go_rule function has been removed. Use rule directly instead.

Remove reference to go_rule from the go_context documentation.

Co-authored-by: Zhongpeng Lin <zplin@uber.com>

go/toolchains.rst

@@ -505,24 +505,14 @@ go_context
 ~~~~~~~~~~
 
 This collects the information needed to form and return a :type:`GoContext` from
-a rule ctx.  It uses the attributes and the toolchains.  It can only be used in
-the implementation of a rule that has the go toolchain attached and the go
-context data as an attribute. To do this declare the rule using the go_rule
-wrapper.
+a rule ctx.  It uses the attributes and the toolchains.
 
 .. code:: bzl
 
   def _my_rule_impl(ctx):
       go = go_context(ctx)
       ...
 
-  my_rule = go_rule(
-      _my_rule_impl,
-      attrs = {
-          ...
-      },
-  )
-
 
 +--------------------------------+-----------------------------+-----------------------------------+
 | **Name**                       | **Type**                    | **Default value**                 |