remove ncurses use flag for mesa llvm dep

It forces pulling in llvm-9999 for me, because llvm-3.3-r1 does not have ncurses USE flag defined. First, it does not seems to be what is intended, second, it made conflits for me:
```
!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

sys-devel/llvm:0

  (sys-devel/llvm-9999::gentoo, ebuild scheduled for merge) pulled in by
    >=sys-devel/llvm-3.3-r1[-ncurses,abi_x86_32(-)?,abi_x86_64(-)?,abi_x86_x32(-)?,abi_mips_n32(-)?,abi_mips_n64(-)?,abi_mips_o32(-)?] required by (media-libs/mesa-9999::gentoo-opencl, ebuild scheduled for merge)

  (sys-devel/llvm-3.3-r2::gentoo, ebuild scheduled for merge) pulled in by
    ~sys-devel/llvm-3.3[clang(-),debug=,multitarget?,python?,static-analyzer,abi_x86_32(-)?,abi_x86_64(-)?,abi_x86_x32(-)?,abi_mips_n32(-)?,abi_mips_n64(-)?,abi_mips_o32(-)?] required by (sys-devel/clang-3.3-r100::gentoo, ebuild scheduled for merge)
```

Note - I don't know what bad effects could be by not disabling ncurses USE flag on newer llvm versions. If it makes problems, some other solution is needed.