doc: develop: tools: drop platformio docs

PlatformIO seems to not keep up with Zephyr (they don't seem to even
support 3.0), so we should not recommend it as an IDE as part of our
official docs. Also, it is frequent to see people reporting problems on
our Discord channels.

Ref. platformio/platform-ststm32#602

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/develop/tools/index.rst

@@ -7,4 +7,3 @@ Tools and IDEs
    :maxdepth: 1
 
    coccinelle.rst
-   platformio/index.rst