Remove PCRE support

[tersec]

It's effectively vestigial.

It's there because https://nim-lang.org/docs/re.html depends on PCRE, but since 0.6.4:

• The PCRE library is no longer necessary for building Nimbus.

This would involve removing it from README.me and

nimbus-build-system/makefiles/variables.mk

Lines 40 to 52 in daff2b4

	# avoid a "libpcre.so.3: cannot open shared object file: No such file or directory" message, where possible
	LINK_PCRE ?= 1
	ifeq ($(LINK_PCRE), 1)
	ifneq ($(OS), Windows_NT)
	ifneq ($(strip $(shell uname)), Darwin)
	ifeq ($(PARTIAL_STATIC_LINKING), 1)
	NIM_PARAMS := $(NIM_PARAMS) -d:usePcreHeader --passL:-l:libpcre.a
	else
	NIM_PARAMS := $(NIM_PARAMS) -d:usePcreHeader --passL:-lpcre
	endif
	endif
	endif
	endif