Disable targets, where we use Nix packages.

--- a/utils/build-presets.ini	2019-04-11 15:19:57.845178834 +0200
+++ b/utils/build-presets.ini	2019-04-11 15:27:42.041297057 +0200
@@ -740,8 +740,6 @@
 llbuild
 swiftpm
 xctest
-libicu
-libcxx
 
 dash-dash
 
@@ -751,9 +749,7 @@
 install-llbuild
 install-swiftpm
 install-xctest
-install-libicu
 install-prefix=%(install_prefix)s
-install-libcxx
 install-sourcekit-lsp
 build-swift-static-stdlib
 build-swift-static-sdk-overlay