Revert "[BPF] support for BPF_ST instruction in codegen"

This reverts commit 92e28e39.

Reverting to investigate buildbot failure reported in [1].

    field-reloc-st-imm.ll:
    *** Bad machine code: Explicit definition must be a register ***
    - function:    bar
    - basic block: %bb.0 entry (0x742f318)
    - instruction: CORE_MEM 3, 416, %0:gpr, @"llvm.foo:0:4$0:2", ...
    - operand 0:   3
    *** Bad machine code: Explicit definition must be a register ***
    - function:    bar
    - basic block: %bb.0 entry (0x742f318)
    - instruction: CORE_MEM 4, 410, %0:gpr, @"llvm.foo:0:8$0:3", ...
    - operand 0:   4
    LLVM ERROR: Found 4 machine code errors.

[1] https://lab.llvm.org/buildbot/#/builders/16/builds/52877