Documentation: riscv: remove non-existent directory from table of contents

This subdir no longer exists: it was there to document some of the
quirks and paths forward for handling them, but those have been dealt
with as of Atish's recent patch set.

Fixes: 23b1f18326 ("Documentation: riscv: Remove the old documentation")
Link: https://lore.kernel.org/linux-next/20220329133412.591d6882@canb.auug.org.au/
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
[Palmer: commit text]
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/index.rst

@@ -7,7 +7,6 @@ RISC-V architecture
 
     boot-image-header
     vm-layout
-    pmu
     patch-acceptance
 
     features