R remove two sides of border from strip.background function

I have the following data:

structure(list(Biological.Group = structure(1:15, .Label = c("A", 
"B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", 
"O"), class = "factor"), Relative.Normalized.Expression....Cq. = c(2L, 
3L, 4L, 6L, 1L, 5L, 7L, 8L, 9L, 3L, 2L, 6L, 7L, 8L, 1L), SE = c(0.171499719, 
0.089692493, 0.153777208, 0.188012958, 0.153776128, 0.192917199, 
0.224766056, 0.231338325, 0.121716906, 0.094763028, 0.09635363, 
0.069986333, 0.113681329, 0.094614957, 0.391182473), Group = structure(c(1L, 
1L, 3L, 3L, 3L, 3L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("", 
"Plant Products", "Sugars"), class = "factor")), class = "data.frame", row.names = c(NA, 
-15L))

This is my code right now (I know it's bulky-- sorry):

>DF <- read.csv(file.choose(), header=TRUE)
>library(ggplot2)
> DFplot= ggplot(DF, aes(Biological.Group,Relative.Normalized.Expression....Cq.))+ylab("Relative Normalized Expression (∆∆Cq)")+geom_bar(fill="black",stat="identity")+  theme(axis.title.x = element_blank())
> DFplot= ggplot(DF, aes(Biological.Group,Relative.Normalized.Expression....Cq.))+ylab("Relative Normalized\nExpression (∆∆Cq)")+geom_bar(fill="black",stat="identity")+  theme(axis.title.x = element_blank())
>DFplot2=DFplot+geom_errorbar(aes(ymin=Relative.Normalized.Expression....Cq.-SE,ymax=Relative.Normalized.Expression....Cq.+SE),width=0.5)+geom_boxplot() + theme(legend.position="none") +ggtitle("MrDF-1")+theme(strip.text = element_text(size=12, face="bold",colour="black"))+scale_y_continuous(expand = c(0,0), labels = scales::number_format(accuracy = 0.1))+theme(panel.border = element_blank(), panel.grid.major = element_blank(), panel.grid.minor = element_blank(), panel.background = element_blank(),axis.line = element_line(colour = "black"))+facet_grid(~Group, scales = "free_x", switch = "x", space = "free_x")+ theme(panel.spacing = unit(1.25, "lines"))+ theme(strip.background = element_rect(color="black", fill="white", size=1.5, linetype="solid"))

That gives me this graph:

I'd like to remove the vertical lines from the strip text labels (specified by strip.background), like this:

I realize I could just use photoshop or something, but I have several graphs to make so it would be easier if I could just specify it in the code.

Any ideas?

Thanks in advance.