misc: pti: Remove a leftover in documentation

Driver is gone, so is the documentation. Remove a leftover in documentation.

Fixes: 8ba59e9dee ("misc: pti: Remove driver for deprecated platform")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210129112729.65363-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/index.rst

@@ -93,7 +93,6 @@ available subsections can be seen below.
    pps
    ptp
    phy/index
-   pti_intel_mid
    pwm
    pldmfw/index
    rfkill