remove riscv64 bits/user.h contents

the contents conflicted with asm/ptrace.h. glibc does not provide
anything in user.h for riscv, so software cannot be depending on it.

simplified from patch submitted by Baruch Siach.

arch/riscv64/bits/user.h

-struct user_regs_struct {
-	unsigned long pc;
-	unsigned long ra;
-	unsigned long sp;
-	unsigned long gp;
-	unsigned long tp;
-	unsigned long t0;
-	unsigned long t1;
-	unsigned long t2;
-	unsigned long s0;
-	unsigned long s1;
-	unsigned long a0;
-	unsigned long a1;
-	unsigned long a2;
-	unsigned long a3;
-	unsigned long a4;
-	unsigned long a5;
-	unsigned long a6;
-	unsigned long a7;
-	unsigned long s2;
-	unsigned long s3;
-	unsigned long s4;
-	unsigned long s5;
-	unsigned long s6;
-	unsigned long s7;
-	unsigned long s8;
-	unsigned long s9;
-	unsigned long s10;
-	unsigned long s11;
-	unsigned long t3;
-	unsigned long t4;
-	unsigned long t5;
-	unsigned long t6;
-};
-
-struct user_fpregs_struct {
-	double f[32];
-	unsigned int fcsr;
-};
-
-#define ELF_NGREG 32
-typedef unsigned long elf_greg_t, elf_gregset_t[ELF_NGREG];
-typedef struct user_fpregs_struct elf_fpregset_t;