Bug#1093961: obsolete build dependencies

[YumeYao]

Source: llvm-toolchain-19
Version: 19.1.7-1

obsolete build dependencies:
libjsoncpp-dev, pkg-config

it's removed in upstream since llvm-7.0.0:
https://github.com/llvm/llvm-project/commit/23655020a271d83870163905905a13a26cfceb94

pkg-config was added along with libjsoncpp-dev according to commit history.

tried building llvm-18 without these dependencies, looked all ok.

applies to llvm 7.0~, if i'm not making mistakes, llvm 18 and 19 are
being actively maintained, so should remove them.

Best Regards,
yumeyao