Revert "[AVR] Include AVR by default in LLVM builds"

This reverts commit dfecec6.

Merging the change revealed that there is a failure on the memory
sanitizer bots.

    Command Output (stderr):
    --
    ==3569==WARNING: MemorySanitizer: use-of-uninitialized-value
        #0 0x1d71bff in
           llvm::AVRSubtarget::ParseSubtargetFeatures(llvm::StringRef,
    llvm::StringRef)
    /b/sanitizer-x86_64-linux-fast/build/llvm_build_msan/lib/Target/AVR/AVRGenSubtargetInfo.inc:471:7
        #1 0x1d721f8 in initializeSubtargetDependencies
           /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Target/AVR/AVRSubtarget.cpp:50:3
        #2 0x1d721f8 in llvm::AVRSubtarget::AVRSubtarget(llvm::Triple
           const&, std::__1::basic_string<char, std::__1::char_traits<char>,
    std::__1::allocator<char> > const&, std::__1::basic_string<char,
    std::__1::char_traits<char>, std::__1::allocator<char> > const&,
    llvm::AVRTargetMachine const&)
    /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Target/AVR/AVRSubtarget.cpp:33:18
        #3 0x1d3077f in
           llvm::AVRTargetMachine::AVRTargetMachine(llvm::Target const&,
    llvm::Triple const&, llvm::StringRef, llvm::StringRef,
    llvm::TargetOptions const&, llvm::Optional<llvm::Reloc::Model>,
    llvm::Optional<llvm::CodeModel::Model>, llvm::CodeGenOpt::Level, bool)
    /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Target/AVR/AVRTargetMachine.cpp:52:7
        #4 0x1d3169d in
           llvm::RegisterTargetMachine<llvm::AVRTargetMachine>::Allocator(llvm::Target
    const&, llvm::Triple const&, llvm::StringRef, llvm::StringRef,
    llvm::TargetOptions const&, llvm::Optional<llvm::Reloc::Model>,
    llvm::Optional<llvm::CodeModel::Model>, llvm::CodeGenOpt::Level, bool)
    /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/include/llvm/Support/TargetRegistry.h:1121:16
        #5 0x86662f in createTargetMachine
           /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/include/llvm/Support/TargetRegistry.h:402:12
        #6 0x86662f in compileModule(char**, llvm::LLVMContext&)
           /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/tools/llc/llc.cpp:473:52
        #7 0x861f42 in main
           /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/tools/llc/llc.cpp:356:22
        #8 0x7f76f7b072e0 in __libc_start_main
           (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
        #9 0x7ebbc9 in _start
           (/b/sanitizer-x86_64-linux-fast/build/llvm_build_msan/bin/llc+0x7ebbc9)

    SUMMARY: MemorySanitizer: use-of-uninitialized-value
    /b/sanitizer-x86_64-linux-fast/build/llvm_build_msan/lib/Target/AVR/AVRGenSubtargetInfo.inc:471:7
    in llvm::AVRSubtarget::ParseSubtargetFeatures(llvm::StringRef,
    llvm::StringRef)
    Exiting
    FileCheck error: '<stdin>' is empty.
    --

The patch wiill be re-committed once fixed.

llvm/CMakeLists.txt

@@ -289,7 +289,6 @@ set(LLVM_ALL_TARGETS
   AArch64
   AMDGPU
   ARM
-  AVR
   BPF
   Hexagon
   Lanai