Remove the useless SRCROOT declaration from the call of build-swig-wrapper-classes.sh & finish-swig-wrapper-classes.sh

Two reasons for that:
* the declaration is not used. the LLDB_SOURCE_DIR is provided as the first argument in the script ($1) (called SRC_ROOT in the source code)
* add_custom_command is quoting the first argument of the command. Usually, it is the script itself (and then the full path to the script) but, here, it is the declaration of a variable.
It was failing with:
cd "/llvm-toolchain-3.3~svn179457/build-llvm/tools/lldb/scripts" && "SRCROOT=/llvm-toolchain-3.3~svn179457/tools/lldb" /llvm-toolchain-3.3~svn179457/tools/lldb/scripts/build-swig-wrapper-classes.sh /llvm-toolchain-3.3~svn179457/tools/lldb /llvm-toolchain-3.3~svn179457/build-llvm/tools/lldb/scripts /llvm-toolchain-3.3~svn179457/build-llvm/tools/lldb/scripts /llvm-toolchain-3.3~svn179457/build-llvm -m
/bin/sh: 1: SRCROOT=/llvm-toolchain-3.3~svn179457/tools/lldb: not found