Remove the old riscv backend

cranelift/filetests/filetests/parser/instruction_encoding.clif

@@ -1,24 +0,0 @@
-test cat
-
-target riscv32
-
-; regex: WS=[ \t]*
-
-function %foo(i32, i32) {
-block1(v0: i32 [%x8], v1: i32):
-    [-,-] v2 = iadd v0, v1
-    [-] trap heap_oob
-    [R#1234, %x5, %x11] v6, v7 = iadd_ifcout v2, v0
-    [Rshamt#beef, %x25] v8 = ishl_imm v6, 2
-@55 v9 = iadd v8, v7
-@a5 [Iret#5] return v0, v8
-}
-; sameln: function %foo(i32, i32) fast {
-; nextln: block1(v0: i32 [%x8], v1: i32):
-; nextln:     [-,-]$WS v2 = iadd v0, v1
-; nextln:     [-]$WS trap heap_oob
-; nextln:     [R#1234,%x5,%x11]$WS v6, v7 = iadd_ifcout v2, v0
-; nextln:     [Rshamt#beef,%x25]$WS v8 = ishl_imm v6, 2
-; nextln:     @0055 [-,-]$WS v9 = iadd v8, v7
-; nextln:     @00a5 [Iret#05]$WS return v0, v8
-; nextln: }