examples: remove Makefile

Back in the days when libgit2 was still young, libgit2 was using plain
Makefiles as build infrastructure. We later changed that to instead use
the CMake build system to make cross-platform development easier. In the
process, we forgot to remove the Makefile from our examples directory,
which is fixed by this commit here. Furthermore, remove the accompanying
.gitignore file, which ignores build outputs. As we do out-of-tree
builds only nowadays, no output is generated in that directory anymore.

examples/.gitignore

-general
-showindex
-diff
-rev-list
-blame
-cat-file
-init
-log
-rev-parse
-remote
-status
-tag
-for-each-ref
-describe
-*.dSYM

examples/Makefile

-.PHONY: all
-
-CC = gcc
-CFLAGS = -g -I../include -I../src -Wall -Wextra -Wmissing-prototypes -Wno-missing-field-initializers
-LFLAGS = -L../build -lgit2 -lz
-APPS = general showindex diff rev-list cat-file status log rev-parse init blame tag remote
-APPS += for-each-ref
-APPS += describe
-
-all: $(APPS)
-
-% : %.c
-	$(CC) -o $@ common.c $(CFLAGS) $< $(LFLAGS)
-
-clean:
-	$(RM) $(APPS)
-	$(RM) -r *.dSYM