configure: Remove pkgconfig macros again (reintroduced by mismerge)

We had removed `PKG_PROG_PKG_CONFIG` in 21b2eba
(bitcoin-core#1090). But then then the not rebased (!) merge of 2be6ba0
(bitcoin-core#1084) brought that macro back at another location, without git
complaining about a conflict.

Fixes bitcoin-core#1127.

configure.ac

@@ -43,8 +43,6 @@ AS_UNSET(ac_cv_prog_AR)
 AS_UNSET(ac_cv_prog_ac_ct_AR)
 LT_INIT([win32-dll])
 
-PKG_PROG_PKG_CONFIG
-
 build_windows=no
 
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