qemu: Remove --cpu= flag

'./configure --cpu=$(uname -m)' (essentially) breaks the case of riscv,
where the configure script expects '--cpu=riscv', but 'uname -m' gives
'riscv{32,64}'. This eventually leads to user-mode emulation being
disabled.

Since the configure script can auto-detect the parameter based on
compilers, remove this flag.

Originally added in #147692, but apparently it's not needed and @NickCao
couldn't remember why it was added. See also mailing list discussion at
https://lore.kernel.org/qemu-devel/20230204112502.2558739-1-mjt@msgid.tls.msk.ru/T/

pkgs/applications/virtualization/qemu/default.nix

@@ -149,7 +149,6 @@ stdenv.mkDerivation rec {
     # have our patches and will be subtly broken because of that.
     "--meson=meson"
     "--cross-prefix=${stdenv.cc.targetPrefix}"
-    "--cpu=${stdenv.hostPlatform.uname.processor}"
     (lib.enableFeature guestAgentSupport "guest-agent")
   ] ++ lib.optional numaSupport "--enable-numa"
     ++ lib.optional seccompSupport "--enable-seccomp"