mk: Remove the concept of 'snapshot transitions'

This way of doing snapshots hasn't been used since 2011.

Makefile.in

@@ -518,18 +518,6 @@ rustc-H-all: $(foreach host,$(CFG_HOST),rustc-H-$(host))
 
 .DEFAULT_GOAL := all
 
-ifneq ($(CFG_IN_TRANSITION),)
-
-CFG_INFO := $(info cfg:)
-CFG_INFO := $(info cfg: *** compiler is in snapshot transition ***)
-CFG_INFO := $(info cfg: *** stage2 and later will not be built ***)
-CFG_INFO := $(info cfg:)
-
-#FIXME This is surely busted
-all: $(SREQ1$(CFG_BUILD)) $(GENERATED) docs
-
-else
-
 define ALL_TARGET_N
 ifneq ($$(findstring $(1),$$(CFG_HOST)),)
 # This is a host
@@ -550,8 +538,6 @@ ALL_TARGET_RULES = $(foreach target,$(CFG_TARGET), \
 
 all: $(ALL_TARGET_RULES) $(GENERATED) docs
 
-endif
-
 
 ######################################################################
 # Re-configuration

configure

@@ -1071,12 +1071,6 @@ then
     putvar CFG_PANDOC
 fi
 
-if head -n 1 ${CFG_SRC_DIR}src/snapshots.txt | grep -q '^T'
-then
-    CFG_IN_TRANSITION=1
-    putvar CFG_IN_TRANSITION
-fi
-
 # Valgrind is only reliable on Linux. On Windows it doesn't work at all, and
 # on the Mac the dynamic linker causes Valgrind to emit a huge stream of
 # errors.