board_misoc: remove pcr

VexRiscv seems to not support additional hardware performance counter, at least I have not seen any documentation on how to use it.

artiq/firmware/ksupport/api.rs

@@ -131,9 +131,6 @@ static mut API: &'static [(&'static str, *const ())] = &[
     api!(cache_get = ::cache_get),
     api!(cache_put = ::cache_put),
 
-    api!(mfspr = ::board_misoc::spr::mfspr),
-    api!(mtspr = ::board_misoc::spr::mtspr),
-
     /* direct syscalls */
     api!(rtio_init = ::rtio::init),
     api!(rtio_get_destination_status = ::rtio::get_destination_status),

artiq/firmware/libboard_artiq/lib.rs

@@ -15,8 +15,6 @@ extern crate io;
 extern crate board_misoc;
 extern crate proto_artiq;
 
-pub mod pcr;
-
 pub mod spi;
 
 #[cfg(has_kernel_cpu)]