fix(compat): drop fgetln compat shim, migrate prompt-history.c to getline

Following up on the comment on #5365 - this drops fgetln from
prompt-history.c (its only remaining caller) in favor of getline, and
removes the fgetln compat shim entirely since nothing references it
anymore: the compat.h declaration, compat/fgetln.c itself, and its
entry in configure.ac's AC_REPLACE_FUNCS list.

fgetln doesn't null-terminate its returned buffer when the final line
in the file lacks a trailing newline, so the old loop had a second
branch to heap-copy and manually terminate that case. getline always
null-terminates regardless, so that branch goes away and the loop
becomes uniform.

Verified with a real build (ubuntu-24.04 container, same
--enable-utf8proc --enable-asan configure CI uses): crafted a history
file with a normal entry, a blank line, and a final entry with no
trailing newline at EOF (the exact case the removed branch existed
for), loaded it through a real ASan-built tmux via show-prompt-history,
all three entries came back correctly. Full regress suite is otherwise
clean against this change.

compat.h

@@ -410,11 +410,6 @@ int		 asprintf(char **, const char *, ...);
 int		 vasprintf(char **, const char *, va_list);
 #endif
 
-#ifndef HAVE_FGETLN
-/* fgetln.c */
-char		*fgetln(FILE *, size_t *);
-#endif
-
 #ifndef HAVE_GETLINE
 /* getline.c */
 ssize_t		 getline(char **, size_t *, FILE *);