Remove extraneous libfolly.pc.in pkg-config file (#1050)

Summary:
- In `c2e51a8e`, i.e.
  Differential Revision: D9213429
  fbshipit-source-id: 849c0fce01d4903d3f8a7b6d196037bf950e5921
  a `libfolly.pc.in` file was placed in the `CMake` directory so that
  the `libfolly.pc` file is installed during CMake configure time.
- Note that the `libfolly.pc.in` in the `folly` directory is not
  referenced anywhere in the build tree, and it has been superseded with
  the one in the `CMake` directory. As such, remove it.
Pull Request resolved: https://github.com/facebook/folly/pull/1050

Reviewed By: calebmarchent

Differential Revision: D14400016

Pulled By: yfeldblum

fbshipit-source-id: 4c8ab4cdad047a4311c84b302b28160dd2bc9ed4

folly/libfolly.pc.in

-prefix=@prefix@
-exec_prefix=@exec_prefix@
-libdir=@libdir@
-includedir=@includedir@
-
-Name: libfolly
-Description: Facebook (Folly) C++ library
-Version: @PKG_VERSION@
-Requires: @PKG_DEPS@
-Libs.private: @PKG_LIBS@
-Libs: -L${libdir} -lfolly
-Cflags: -I${includedir} @PKG_CXXFLAGS@