README: do not mention /usr/include on macOS

For newer versions of Xcode, "xcode-select --install" no longer installs
system headers into /usr/include (instead, they are installed in the
Xcode directory tree), so do not mention that path in the macOS section
of README to prevent confusion.

(cherry picked from commit 5af0b1d1)

README

@@ -340,8 +340,6 @@ macOS
 Building on macOS assumes that the "Command Tools for Xcode" is installed.
 This can be downloaded from https://developer.apple.com/download/more/ or
 if you have Xcode already installed you can run "xcode-select --install".
-This will add /usr/include to the system and install the compiler and
-other tools so that they can be easily found.
 
 Dependencies
 

README.md

@@ -349,8 +349,6 @@ affect compilation:
 Building on macOS assumes that the "Command Tools for Xcode" is installed.
 This can be downloaded from https://developer.apple.com/download/more/
 or if you have Xcode already installed you can run "xcode-select --install".
-This will add /usr/include to the system and install the compiler and other
-tools so that they can be easily found.
 
 ### <a name="dependencies"/> Dependencies