[Specs] Remove all the obsolete specs.

sky/specs/README.md

-The Sky Environment
-===================
-
-The main files loaded by the Sky environment are Sky files, though
-they can refer to binary resources like images and fonts.
-
-Sky files
----------
-
-Conventional MIME type: ``text/sky``, though this type is neither
-necessary nor sufficient to indicate that a file is a Sky file; only
-the signature matters for type dispatch of Sky files.
-
-Conventional extension: ``.sky``
-
-Signatures:
-
-For application files, one of the following:
-* ``23 21 6d 6f 6a 6f 20 6d 6f 6a 6f 3a 73 6b 79 0a`` ("``#!mojo mojo:sky\n``")
-* ``23 21 6d 6f 6a 6f 20 6d 6f 6a 6f 3a 73 6b 79 0d`` ("``#!mojo mojo:sky\r``")
-* ``23 21 6d 6f 6a 6f 20 6d 6f 6a 6f 3a 73 6b 79 20`` ("``#!mojo mojo:sky ``")
-
-For module files, one of the following:
-* ``53 4b 59 20 4d 4f 44 55 4c 45 0a`` ("``SKY MODULE\n``")
-* ``53 4b 59 20 4d 4f 44 55 4c 45 0d`` ("``SKY MODULE\r``")
-* ``53 4b 59 20 4d 4f 44 55 4c 45 20`` ("``SKY MODULE ``")
-
-
-Notes
------
-
-```
-magical imports:
-  the mojo fabric API   dart:mojo
-  the mojom for the shell, proxying through C++ so that the shell pipe isn't exposed  dart:mojo-shell
-  the Sky API  dart:sky
-```
+This file contains documentation for what we hope Sky to support in due course.
+It's mostly proposals. It's not intended to be comprehensive.