[cmake] Remove old unused version of FindZ3.cmake from clang [NFC]

Summary: This file was moved to llvm in D54978, r356929, but the old
file was never removed.

Reviewed By: beanz

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62343

llvm-svn: 361663
diff --git a/clang/cmake/modules/FindZ3.cmake b/clang/cmake/modules/FindZ3.cmake
deleted file mode 100644
index e69de29..0000000
--- a/clang/cmake/modules/FindZ3.cmake
+++ /dev/null