cmake: don't pass ccache argument to RocksDB build

cmake: don't pass ccache argument to RocksDB build

RocksDB's cmake has a similar logic to turn on ccache as the main cmake.
Currently BuildRocksDB.cmake explicitly passes ccache to RocksDB cmake
arguments as CMAKE_CXX_COMPILER_LAUNCHER. This causes a compilation error
on ccache 4.0, especially in make -j mode.

ccache: error: Recursive invocation (the name of the ccache binary must
be "ccache")

This commit lets cmake from RocksDB decide RULE_LAUNCH_COMPILE by itself.

Signed-off-by: Sheng Mao <shngmao@gmail.com>