Remove theorem prover targets from default Makefile rule (#464)

In general we aren't requiring contributors to implement the correct
Lem/Isabelle/HOL4/Coq stubs for new extensions (this would almost
certainly be way too high a bar) so having these in the default set of
build targets just means that typing 'make' is broken until those of us
who are invested in maintaining those targets can add updates for those
stubs.

Makefile

@@ -201,7 +201,7 @@ RISCV_EXTRAS_LEM = $(addprefix handwritten_support/,$(RISCV_EXTRAS_LEM_FILES))
 
 .PHONY:
 
-all: ocaml_emulator/riscv_ocaml_sim_$(ARCH) c_emulator/riscv_sim_$(ARCH) riscv_isa riscv_coq riscv_hol riscv_rmem
+all: ocaml_emulator/riscv_ocaml_sim_$(ARCH) c_emulator/riscv_sim_$(ARCH)
 .PHONY: all
 
 # the following ensures empty sail-generated .c files don't hang around and