cmake: drop redundant -lws2_32 meant for libssh2.pc

The `libssh2.pc` generator logic automatically adds `-lws2_32` while
parsing `LIBSSH2_LIBS`, which contains this lib already. Then discard
the duplicate.

This patch introduces a change in the position of `ws2_32` within
the lib list advertised via `libssh2.pc` for static builds.

This order might in cases by significant, but:

- libssh2 no longer links against `libssl`, which was the library
  also referencing `ws2_32` and breaking picky binutils `ld` linker
  when not passed in strict dependency order.
  Ref: c84745e34e #1128

- since switching to INTERFACE targets, cmake messes up the lib order
  anyway, adding `OpenSSL:Crypto` last, instead of `ws2_32`. This did
  not seem to cause an issue so far.
  Ref: df0563a857 #1535

For these reasons it seems unlikely this position change could break
Windows OpenSSL static gcc/ld builds relying on `libssh2.pc` for their
lib list. It least no more than it was before this patch.

Turns out these theories don't stand in practice and the order is broken
possibly by introducing INTERFACE targets, with or without this patch.
`libcrypto` (tested with LibreSSL) is in fact depending on `ws2_32`, and
the `bcrypt` dependency is also causing breakage with picky binutils ld.
I may try addressing it in a separate PR.

Ref: 33b6d5f89d #827
Ref: 31fb8860db #811

Closes #1619

src/CMakeLists.txt

@@ -229,10 +229,6 @@ install(
 # Calculate variables for pkg-config
 set(LIBSSH2_PC_LIBS_PRIVATE "")
 
-if(WIN32)
-  list(APPEND LIBSSH2_PC_LIBS_PRIVATE "-lws2_32")
-endif()
-
 set(_ldflags "")
 
 # Avoid getting unnecessary -L options for known system directories.