BREAKING - "i2s.h" removed, I2S library added

An "i2S.h" header was presesnt in prior cores, but this conflicts with the
Arduino standard I2S.h header for the I2S class under Windows (because of
case insensitivity).

Initial 3.0.0 release has a redirect "i2s.h" file in the cores directory to
redirect to the "I2S.h" file in the library, but under Windows this resulted
in the IDE not building the I2S library and link errors.

Remove the offending header.  Code will need to mode to including "I2S.h"
instead (which will include both the Arduino class as well as the low-level
definitions).

Fixes esp8266#8107